Title of Invention: Multi-Stage Process for Producing a Material of a Battery Cell Abstract: A system and method thereof are provided for multi-stage processing of one or more precursor compounds into a battery material. The system includes a mist generator, a drying chamber, one or more gas-solid separators, and one or more in-line reaction modules comprised of one or more gas-solid feeders, one or more gas-solid separators, and one or more reactors. Various gas-solid mixtures are formed within the internal plenums of the drying chamber, the gas-solid feeders, and the reactors. In addition, heated air or gas is served as the energy source within the processing system and as the gas source for forming the gas-solid mixtures to facilitate reaction rate and uniformity of the reactions therein. Precursor compounds are continuously delivered into the processing system and processed in-line through the internal plenums of the drying chamber and the reaction modules into final reaction particles useful as a battery material. Detailed Description: CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation of U.S. patent application Ser. No. 13/901,035, filed May 23, 2013, which claims benefit of U.S. provisional patent application Ser. No. 61/855,063, filed May 6, 2013. All of the above-referenced applications are herein incorporated by reference. FIELD OF THE INVENTION This invention generally relate to the preparation of materials for battery applications. More specifically, the invention related to method and system in manufacturing structured cathode or anode active materials for use in secondary batteries. BACKGROUND OF THE INVENTION Great efforts have been devoted to the development of advanced electrochemical battery cells to meet the growing demand of various consumer electronics, electrical vehicles and grid energy storage applications in terms of high energy density, high power performance, high capacity, long cycle life, low cost and excellent safety. In most cases, it is desirable for a battery to be miniaturized, light-weighted and rechargeable (thus reusable) to save space and material resources. In an electrochemically active battery cell, a cathode and an anode are immersed in an electrolyte and electronically separated by a separator. The separator is typically made of porous polymer membrane materials such that metal ions released from the electrodes into the electrolyte can diffuse through the pores of the separator and migrate between the cathode and the anode during battery charge and discharge. The type of a battery cell is usually named from the metal ions that are transported between its cathode and anode electrodes. Various rechargeable secondary batteries, such as nickel cadmium battery, nickel-metal hydride battery, lead acid battery, lithium ion battery, and lithium ion polymer battery, etc., have been developed commercially over the years. To be used commercially, a rechargeable secondary battery is required to be of high energy density, high power density and safe. However, there is a trade-off between energy density and power density. Lithium ion battery is a secondary battery which was developed in the early 1990s. As compared to other secondary batteries, it has the advantages of high energy density, long cycle life, no memory effect, low self-discharge rate and environmentally benign. Lithium ion battery rapidly gained acceptance and dominated the commercial secondary battery market. However, the cost for commercially manufacturing various lithium battery materials is considerably higher than other types of secondary batteries. In a lithium ion battery, the electrolyte mainly consists of lithium salts (e.g., LiPF6, LiBF4 or LiClO4) in an organic solvent (e.g., ethylene carbonate, dimethyl carbonate, and diethyl carbonate) such that lithium ions can move freely therein. In general, aluminum foil (e.g., 15˜20 μm in thickness) and copper foil (e.g., 8˜15 μm in thickness) are used as the current collectors of the cathode electrode and the anode electrode, respectively. For the anode, micron-sized graphite (having a reversible capacity around 330 mAh/g) is often used as the active material coated on the anode current collector. Graphite materials are often prepared from solid-state processes, such as grinding and pyrolysis at extreme high temperature without oxygen (e.g., graphitization at around 3000° C.). As for the active cathode materials, various solid materials of different crystal structures and capacities have been developed over the years. Examples of good cathode active materials include nanometer- or micron-sized lithium transition metal oxide materials and lithium ion phosphate, etc. Cathode active materials are the most expensive component in a lithium ion battery and, to a relatively large extent, determines the energy density, cycle life, manufacturing cost and safety of a lithium battery cell. When lithium battery was first commercialized, lithium cobalt oxide (LiCoO2) material is used as the cathode material and it still holds a significant market share in the cathode active material market. However, cobalt is toxic and expensive. Other lithium transition metal oxide materials, such as layered structured LiMeO2 (where the metal Me═Ni, Mn, Co, etc.; e.g., LiNi0.33Mn0.33Co0.33O2, with their reversible/practical capacity at around 140˜150 mAh/g), spinel structured LiMn2O4 (with reversible/practical capacity at around 110˜120 mAh/g), and olivine-type lithium metal phosphates (e.g., LiFePO4, with reversible/practical capacity at around 140˜150 mAh/g) have recently been developed as active cathode materials. When used as cathode materials, the spinel structured LiMn2O4 materials exhibit poor battery cycle life and the olivine-type LiFePO4 materials suffer from low energy density and poor low temperature performance. As for LiMeO2 materials, even though their electrochemical performance is better, prior manufacturing processes for LiMeO2 can obtain mostly agglomerates, such that the electrode density for most LiMeO2 materials is lower as compared to LiCoO2. In any case, prior processes for manufacturing materials for battery applications, especially cathode active materials, are too costly as most processes consumes too much time and energy, and still the qualities of prior materials are inconsistent and manufacturing yields are low. Conventional material manufacturing processes such as solid-state reaction (e.g., mixing solid precursors and then calcination) and wet-chemistry processes (e.g., treating precursors in solution through co-precipitation, sol-gel, or hydrothermal reaction, etc., and then mixing and calcination) have notable challenges in generating nano- and micron-structured materials. It is difficult to consistently produce uniform solid materials (i.e., particles and powders) at desired particle sizes, morphology, crystal structures, particle shape, and even stoichiometry. Most conventional solid-state reactions require long calcination time (e.g., 4-20 hours) and additional annealing process for complete reaction, homogeneity, and grain growth. For example, spinel structured LiMn2O4 and olivine-type LiFePO4 materials manufactured by solid-state reactions require at least several hours of calcination, plus a separate post-heating annealing process (e.g., for 24 hours), and still showing poor quality consistency. One intrinsic problem with solid-state reaction is the presence of temperature and chemical (such as O2) gradients inside a calcination furnace, which limits the performance, consistency and overall quality of the final products. On the other hand, wet chemistry processes performed at low temperature usually involve faster chemical reactions, but a separate high temperature calcination process and even additional annealing process are still required afterward. In addition, chemical additives, gelation agents, and surfactants required in a wet chemistry process will add to the material manufacturing cost (in buying additional chemicals and adjusting specific process sequence, rate, pH, and temperature) and may interfere with the final composition of the as-produced active materials (thus often requiring additional steps in removing unwanted chemicals or filtering products). Moreover, the sizes of the primary particles of the product powders produced by wet chemistry are very small, and tend to agglomerates into undesirable large sized secondary particles, thus affecting energy packing density. Also, the morphologies of the as-produced powder particles often exhibit undesirable amorphous aggregates, porous agglomerates, wires, rods, flakes, etc. Uniform particle sizes and shapes allowing for high packing density are desirable. The synthesis of lithium cobalt oxide (LiCoO2) materials is relatively simple and includes mixing a lithium salt (e.g., lithium hydroxide (LiOH) or lithium carbonate (Li2CO3)) with cobalt oxide (Co3O4) of desired particle size and then calcination in a furnace at a very high temperature for a long time (e.g., 20 hours at 900° C.) to make sure that lithium metal is diffused into the crystal structure of cobalt oxide to form proper final product of layered crystal structured LiCoO2 powders. This approach does not work for LiMeO2 since transition metals like Ni, Mn, and Co does not diffuse well into each other to form uniformly mixed transition metal layers if directly mixing and reacting (solid-state calcination) their transition metal oxides or salts. Therefore, conventional LiMeO2 manufacturing processes requires buying or preparing transitional metal hydroxide precursor compounds (e.g., Me(OH)2, Me═Ni, Mn, Co, etc.) from a co-precipitation wet chemistry process prior to making final active cathode materials (e.g., lithium NiMnCo transitional metal oxide (LiMeO2)). Since the water solubility of these Ni(OH)2, Co(OH)2, and Mn(OH)2 precursor compounds are different and they normally precipitate at different concentrations, the pH of a mixed solution of these precursor compounds has to be controlled and ammonia (NH3) or other additives has to be added slowly and in small aliquots to make sure nickel (Ni), manganese (Mn), and cobalt (Co) can co-precipitate together to form micron-sized nickel-manganese-cobalt hydroxide (NMC(OH)2) secondary particles. Such co-precipitated NMC(OH)2 secondary particles are often agglomerates of nanometer-sized primary particles. Therefore, the final lithium NMC transitional metal oxide (LiMeO2) made from NMC(OH)2 precursor compounds are also agglomerates. These agglomerates are prone to break under high pressure during electrode calendaring step and being coated onto a current collector foil. Thus, when these lithium NMC transitional metal oxide materials are used as cathode active materials, relatively low pressure has to be used in calendaring step, and further limiting the electrode density of a manufactured cathode. In conventional manufacturing process for LiMeO2 active cathode materials, precursor compounds such as lithium hydroxide (LiOH) and transitional metal hydroxide (Me(OH)2 are mixed uniformly in solid-states and stored in thick Al2O3 crucibles. Then, the crucibles are placed in a heated furnace with 5-10° C./min temperature ramp up speed until reaching 900° to 950° C. and calcinated for 10 to 20 hours. Since the precursor compounds are heated under high temperature for a long time, the neighboring particles are sintered together, and therefore, a pulverization step is often required after calcination. Thus, particles of unwanted sizes have to be screened out after pulverization, further lowering down the overall yield. The high temperature and long reaction time also lead to vaporization of lithium metals, and typically requiring as great as 10% extra amount of lithium precursor compound being added during calcination to make sure the final product has the correct lithium/transition metal ratio. Overall, the process time for such a multi-step batch manufacturing process will take up to a week so it is very labor intensive and energy consuming. Batch process also increases the chance of introducing impurity with poor run-to-run quality consistency and low overall yield. Thus, there is a need for an improved process and system to manufacture high quality, structured active materials for a battery cell. SUMMARY OF THE INVENTION This invention generally relate to preparing materials for battery applications. More specifically, the invention related to method and system for producing material particles (e.g., active electrode materials, etc.) in desirable crystal structures, sizes and morphologies. In one embodiment, a multi-stage in-line processing system and method thereof is provided for producing a material of a battery cell. The processing system generally includes one or more processing modules comprised of a mist generator, a drying chamber, one or more gas-solid separators, and one or more in-line reaction modules. The reaction modules include one or more gas-solid feeders and/or one or more reactors, and optionally one or more gas-solid separators. The processing modules and the reaction modules are provided for multi-stage processing of one or more precursor compounds into final reaction product particles. In another embodiment, one or more cooling mechanisms are provided to lower the temperature of final reaction product particles. Various gas-solid mixtures are formed within the internal plenums of the drying chamber, the one or more gas-solid feeders, and the one or more reactors. In addition, heated air or gas is served as the energy source for reactions inside the drying chamber, the gas-solid feeders, and/or the reactors and as the gas source for forming the gas-solid mixtures to facilitate reaction rate and uniformity of the reactions therein. Solid precursors are continuously delivered into the processing modules and the reaction modules of the processing system and processed, through various plenums of the chambers, feeder reactors, and/or reactors within the processing system, into final reaction product particles. In one embodiment, a method of producing a material (e.g., cathode or anode active materials) for a battery electrochemical cell is provided. The method includes drying a first mixture formed from a mist of a liquid mixture comprising one or more precursors and a flow of a first gas at a first temperature for a first residence time inside a drying chamber, and separating the first mixture into a first type of solid particles and a first side product. Next, the first type of solid particles of one or more precursor compounds are delivered through one or more multi-stage in-line reaction modules for further reaction. Within a first reaction module, a second gas-solid mixture is formed from the first type of solid particles and a flow of a second gas heated to a second temperature and the second gas-solid mixture is separated into a second type of solid particles and a second side product. The second type of solid particles is then delivered into a second reaction module to form a third gas-solid mixture comprising the second type of solid particles and a flow of a third gas heated to a third temperature inside a reactor. Then, the third gas-solid mixture is reacted for a second residence time inside the reactor and a reacted gas-solid mixture is formed. The reacted gas-solid mixture is cooled to obtain final reacted solid particles. In one aspect, the final reacted solid particles are suitable as an active electrode material to be further processed into an electrode of a battery cell. In another aspect, the reacted gas-solid mixture is separated into a third type of solid particles and a third side product, and the third type of solid particles is further processed into a battery material. In still another aspect, one or more flows of a cooling fluid can be used to cool the temperature of the final reacted solid particles and/or the third type of solid particles. In a further aspect, a flow of a cooling fluid is delivered to be mixed with the final reacted solid particles to form a cooled gas solid-mixture and cool the temperature thereof. The cooled gas-solid mixture is then separated into a cooled final reacted solid particles and a fourth side product. In another embodiment, a method of producing a material for a battery electrochemical cell incudes drying a first mixture formed from a mist of a liquid mixture and a flow of a first gas at a first temperature for a first residence time inside a drying chamber, and separating the first mixture into a first type of solid particles and a first side product. Next, the first type of solid particles is delivered through one or more multi-stage in-line reaction modules of a processing system for further reaction. The method further includes forming a second gas-solid mixture in a first reaction module and forming a third gas-solid module in a second reaction module. Within the first reaction module, the second gas-solid mixture formed from the first type of solid particles and a flow of a second gas heated to a second temperature is reacted for a second residence time and separated into a second type of solid particles and a second side product. Next, the second type of solid particles is delivered continuously into a second reaction module. Within the second reaction module, the third gas-solid mixture formed from the second type of solid particles and a flow of a third gas heated to a third temperature is reacted inside a reactor for a third residence time. Then, a portion of gas-solid mixtures within the reactor is delivered out of the reactor and separated into a third type of solid particles and a third side product. The method further includes circulating a portion of the third type of solid particles back into the reactor to be reacted for a fourth residence time inside the reactor and forming a reacted gas-solid mixture, and separating the reacted gas-solid mixture into final reacted solid particles and a fourth side product. Optionally, a flow of a cooling fluid is delivered to mix with the final reacted solid particles and form a cooled gas solid-mixture. The cooled gas-solid mixture can then be separated into a cooled final reacted solid particles and a fifth side product. In still another embodiment, a method of producing a material for a battery electrochemical cell includes drying a first mixture formed from a mist of a liquid mixture and a flow of a first gas inside a drying chamber at a first temperature for a first residence time, separating the first mixture into a first type of solid particles and a first side product, reacting a second mixture formed from the first type of solid particles and a flow of a second gas heated to a second temperature inside a gas-solid feeder for a second residence time, and separating the second mixture into a second type of solid particles and a second side product. Next, the second type of solid particles is delivered to a fluidized bed reactor and mixed with a flow of a third gas that is heated to a third temperature to form a third mixture. The fluidized bed reactor may be a circulating fluidized bed reactor, a bubbling fluidized bed reactor, an annular fluidized bed reactor, a flash fluidized bed reactor, and combinations thereof. Inside the fluidized bed reactor, the third mixture is reacted for a third residence time to form a reacted gas-solid mixture. Then, the reacted gas-solid mixture is delivered out of the fluidized bed reactor. The reacted gas-solid mixture can be further processed and/or cooled to obtain final reacted solid particles. In one aspect, the final reacted solid particles are mixed with a flow of a cooling fluid to form a cooled gas solid-mixture from and cool the temperature of the final reacted solid particles. The cooled gas-solid mixture is then separated into a cooled final reacted solid particles and a third side product. In a further embodiment, a multi-stage in-line processing system for manufacturing a material of a battery cell is provided. The processing system includes a drying chamber connected to a first gas line and adapted to flow a first gas inside the drying chamber, and a first gas-solid separator connected to the drying chamber, wherein the first gas-solid separator receives a chamber-product from the drying chamber and separates the one or more drying chamber products into a first type of solid particles and a first side product. The processing system further includes one or more gas-solid feeders, one or more second gas-solid separators, and one or more reactors. The one or more gas-solid feeders are connected to the first gas-solid separator and one or more second gas lines, wherein the one or more gas-solid feeders receive the first type of solid particles from the first gas-solid separator, mix a second gas with the first type of solid particles, and form one or more gas-solid mixtures therein. The one or more second gas-solid separators are connected to the one or more gas-solid feeders, wherein the one or more second gas-solid separators separate the one or more gas-solid mixtures into one or more types of solid particles and one or more side products. The one or more reactors are connected to the one or more second gas-solid separators and a third gas line having a third gas flowed therein, wherein the one or more reactors receive the one or more types of solid particles from the one or more second gas-solid separators and mix the one or more types of solid particles with the third gas into a reaction mixture, wherein a final reaction product is obtained from a reaction of the reaction mixture within the one or more reactors. In one aspect, the processing system may further include a mist generator connected to the drying chamber and adapted to generate a mist from a liquid mixture of one or more precursors. In another aspect, the processing system uses heated gas being pre-heated to a desired temperature and flowed from one or more gas lines into the processing system as energy source to react various gas-solid mixtures into final reaction products. In still another aspect, the multi-stage in-line processing system provides a first stage of processing the one or more precursors into the first type of solid particles using the drying chamber and the first gas-solid separator. Next, the multi-stage in-line processing system provides one or more in-line reaction modules to process the first type of solid particles containing the one or more precursors into final reaction products. For example, a first reaction module within the processing system may include one or more gas-solid feeders and one or more second gas-solid separators to process the first type of solid particles into one or more types of solid particles; whereas a second reaction module may include one or more reactors to process the one or more types of solid particles into the final reaction product. Within the first reaction module, the gas-solid feeders are connected to the first gas-solid separator and one or more second gas lines to receive the first type of solid particles from the first gas-solid separator, mix a second gas with the first type of solid particles, and form one or more gas-solid mixtures therein. The one or more second gas-solid separators are connected to the one or more gas-solid feeders to separate the one or more gas-solid mixtures into one or more types of solid particles and one or more side products. Within the second reaction module, one or more reactors (e.g., a fluidized bed reactor) are used to receive the one or more types of solid particles from the one or more second gas-solid separators and mix the one or more types of solid particles with a third gas into a reaction mixture. A final reaction product is obtained from a reaction of the reaction mixture within the fluidized bed reactor. In still another embodiment, a processing system of producing a material for a battery cell includes a drying chamber, a first gas-solid separator, one or more gas-solid feeders, one or more second gas-solid separators, and a fluidized bed reactor. In one aspect, the processing system further comprises one or more third gas-solid separators connected to the fluidized bed reactor and adapted to separate a portion of a reaction mixture from the fluidized bed reactor into solid particles and deliver a portion of the solid particles back into the fluidized bed reactor for further reaction. In another aspect, the processing system further comprises one or more cooling mechanism adapted to cool a final reaction product obtained from a reaction of the reaction mixture. The one or more cooling mechanisms may be one or more gas-solid separators, gas-solid feeders, heat exchangers, fluidized beds, and combinations thereof. In still another embodiment, a processing system of producing a material for a battery cell includes a first module comprising a drying chamber and a first gas-solid separator. The drying chamber includes a chamber inlet adapted to deliver a mist of a precursor-containing liquid mixture, and a gas inlet connected to a first gas line and adapted to flow a first gas inside the drying chamber. The first gas-solid separator is connected to the drying chamber to receive a chamber-product from the drying chamber and separate the one chamber-product into a first type of solid particles and a first side product. The processing system further includes one or more second modules, where each second module includes a first gas-solid feeder and a second gas-solid separator. The first gas-solid feeder may include a feeder inlet connected to the first gas-solid separator and adapted to receive the first type of solid particles from the first gas-solid separator, and a feeder gas inlet connected to a second gas line and adapted to flow a second gas to be mixed with the first type of solid particles and form a second gas-solid mixture therein. The second gas-solid separator is connected to the first gas-solid feeder to separate the one or more gas-solid mixtures into a second type of solid particles and a second side product. The processing system further includes a third module comprising a reactor, where a final reaction product is obtained from a reaction of reaction mixtures within the reactor. The reactor may include a reactor inlet and a reactor gas inlet. The reactor inlet is connected to the second gas-solid separator and adapted to receive the second type of solid particles from the second gas-solid separator. The reactor gas inlet is connected to a third gas line and adapted to flow a third gas to be mixed with the second type of solid particles into reaction mixtures. Optionally, the processing system further includes one or more gas-solid separators and a cooling module comprising one or more cooling mechanisms. At least one of the one or more gas-solid separators may be connected to the reactor and adapted to separate a portion of the reaction mixture into a third type of solid particles and deliver a portion of the third type of .solid particles back into the reactor for further reaction. BRIEF DESCRIPTION OF THE DRAWINGS So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. FIG. 1A illustrates one embodiment of a flow chart of a method of producing a material for a battery electrochemical cell. FIG. 1B illustrates a continuation of the flow chart of FIG. 1A. FIG. 2A is a flow chart illustrating one embodiment of employing a processing system in a method of preparing a battery material. FIG. 2B illustrates a multi-stage in-line processing system useful in preparing a battery material according one embodiment of the invention. FIG. 3A is a schematic of one embodiment of a processing system useful in preparing a material for a battery electrochemical cell. FIG. 3B is a schematic of another embodiment of a processing system useful in preparing a material for a battery electrochemical cell. FIG. 3C is a schematic of still another embodiment of a processing system useful in preparing a material for a battery electrochemical cell. FIG. 3D is a schematic of yet another embodiment of a processing system useful in preparing a material for a battery electrochemical cell. FIG. 4A illustrates one example of a temperature-versus-time profile of a multi-stage process using the processing system of FIG. 3A to prepare a battery material according to one embodiment of the invention. FIG. 4B illustrates another example of a temperature-versus-time profile of a multi-stage process using the processing system of FIG. 3B to prepare a battery material according to one embodiment of the invention. FIG. 4C illustrates another example of a temperature-versus-time profile of a multi-stage process using the processing system of FIG. 3C to prepare a battery material according to one embodiment of the invention. FIG. 4D illustrates another example of a temperature-versus-time profile of a multi-stage process using the processing system of FIG. 3D to prepare a battery material according to one embodiment of the invention. FIG. 5A illustrates one exemplary gas-solid separator useful in an in-line processing system to prepare a material of a battery cell according various embodiments of the invention. FIG. 5B illustrates another exemplary gas-solid separator useful in an in-line processing system to prepare a material of a battery cell according various embodiments of the invention. FIG. 5C illustrates still another exemplary gas-solid separator useful in an in-line processing system to prepare a material of a battery cell according various embodiments of the invention. FIG. 6A illustrates one exemplary reactor useful in an in-line processing system to prepare a battery material according one embodiment of the invention. FIG. 6B illustrates another exemplary reactor useful in an in-line processing system to prepare a battery material according one embodiment of the invention. FIG. 6C illustrates still another exemplary reactor useful in an in-line processing system to prepare a battery material according one embodiment of the invention. FIG. 6D illustrates yet another exemplary reactor useful in an in-line processing system to prepare a battery material according one embodiment of the invention. FIGS. 7A-7F show a schematics of a drying chamber with exemplary gas flows and mist flows therein according to various embodiments of the invention. DETAILED DESCRIPTION The present invention generally provides a multi-stage in-line processing system and a method thereof for producing a material of a battery cell. The processing system generally includes a first-stage processing module, one or more second-stage reaction modules, a third-stage reaction module, and a fourth-stage cooling module. Solid precursors are continuously delivered into the processing system and processed, through various plenums of the in-line processing and reaction modules, into final reaction particles. For example, the processing system may include a mist generator, a drying chamber, one or more gas-solid separators, one or more gas-solid feeders, a reactor, and one or more cooling mechanisms. Various gas-solid mixtures are formed within the internal plenums of the drying chamber, the one or more gas-solid feeders, and the reactor. In addition, heated air or gas is served as the energy source for any reactions inside the drying chamber, the gas-solid feeders, and/or the reactor, and as the gas source for forming the gas-solid mixtures to facilitate reaction rates and uniformity of the reactions therein. The processing system is useful in performing a continuous process to manufacture a material for a battery cell, save material manufacturing time and energy, and solve the problems of high manufacturing cost, low yield, poor quality consistency, low electrode density, low energy density as seen in conventional active material manufacturing processes. In one aspect, one or more precursor compounds, such as one or more metal-containing precursors, are mixed with a liquid solvent to form a liquid mixture to obtain uniform blending of the precursors. In another aspect, the ratio of different metal precursors within the liquid mixture can be adjusted (e.g., in desirable molar ratio that correlated to desired composition of the final reaction products). The liquid mixture is then promptly dried into a first gas-solid mixture, which includes dried solid particles of the precursors evenly distributed in a gas phase. The first gas-solid mixture is then separated by a first gas-solid separator into a first type of solid particles containing the evenly mixed precursors. Next, the first type of solid particles is continuously delivered into one or more in-line reaction modules for further reaction. For example, a first reaction module may be used to process the first type of solid particles into partially-reacted product particles, which are then processed by a second reaction module into final reaction products. The final reaction products are then cooled by a cooling module comprising one or more cooling mechanisms. In still another aspect, gas-solid mixtures are formed inside the drying chamber and the one or more processing and reaction modules, and are further separated into solid particles to be delivered into the next processing modules. Unwanted waste and side products and reaction by-products are separated and removed during the continuous material manufacturing process to ensure the quality of final product particles. Accordingly, a continuous multi-stage process is performed within the processing system to obtain high quality and consistent active battery materials with much less time, labor, and supervision than materials prepared from conventional manufacturing processes. FIG. 1 illustrates a method 100 of producing a material useful in a battery electrochemical cell. Firstly, at step 102 of the method 100, a flow of a first gas is flowed in to a drying chamber, and a mist is also generated inside the drying chamber. The mist is formed from a liquid mixture comprised of one or more precursors dissolved and/or dispersed in a suitable liquid solvent. Within the drying chamber, the flow of the first gas and the flow of the mist are mixed together. In general, liquid form of a precursor compound can be prepared directly into a liquid mixture in a desired concentration. Solid form of a precursor compound can be dissolved or dispersed in a suitable solvent (e.g., water, alcohol, isopropanol, or any other organic or inorganic solvents, and their combinations) to form into a liquid mixture of an aqueous solution, slurry, gel, aerosol or any other suitable liquid forms. One or more precursors can be used, depending on the desired composition of a final reaction product. For example, two or more solid precursors can be prepared in desirable molar ratio and mixed into a liquid mixture, such as by measuring and preparing appropriate amounts of the two or more solid precursors into a container with suitable amounts of a solvent. Depending on the solubility of the precursors in a chosen solvent, pH, temperature, and mechanical stirring and mixing can be adjusted to obtain a liquid mixture where one or more precursor compounds at the desirable molar concentrations are fully dissolved and/or evenly dispersed. In one example, two or more metal-containing precursors are mixed into a liquid mixture for obtaining a final reaction product of a mixed metal oxide material. Exemplary metal-containing precursors include, but are not limited to, metal salts, lithium-containing compound, cobalt-containing compound, manganese-containing compound, nickel-containing compound, lithium sulfate (Li2SO4), lithium nitrate (LiNO3), lithium carbonate (Li2CO3), lithium acetate (LiCH2COO), lithium hydroxide (LiOH), lithium formate (LiCHO2), lithium chloride (LiCl), cobalt sulfate (CoSO4), cobalt nitrate (Co(NO3)2), cobalt carbonate (CoCO3), cobalt acetate (Co(CH2COO)2), cobalt hydroxide (Co(OH)2), cobalt formate (Co(CHO2)2), cobalt chloride (CoCl2), manganese sulfate (MnSO4), manganese nitrate (Mn(NO3)2), manganese carbonate (MnCO3), manganese acetate (Mn(CH2COO)2), manganese hydroxide (Mn(OH)2), manganese formate (Mn(CHO2)2), manganese chloride (MnCl2), nickel sulfate (NiSO4), nickel nitrate (Ni(NO3)2), nickel carbonate (NiCO3), nickel acetate (Ni(CH2COO)2), nickel hydroxide (Ni(OH)2), nickel formate (Ni(CHO2)2), nickel chloride (NiCl2), aluminum (Al)-containing compound, titanium (Ti)-containing compound, sodium (Na)-containing compound, potassium (K)-containing compound, rubidium (Rb)-containing compound, vanadium (V)-containing compound, cesium (Cs)-containing compound, chromium (Cr)-containing compound, copper (Cu)-containing compound, magnesium (Mg)-containing compound, iron (Fe)-containing compound, and combinations thereof, among others. Not wishing to be bound by theory, it is contemplated that, in order to prepare an oxide material with two or more different metals, all of the required metal elements are first mixed in liquid phase (e.g., into a solution, slurry, or gel) using two or more metal-containing precursor compounds as the sources of each metal element such that the two or more different metals can be mixed uniformly at desired ratio. As an example, to prepare a liquid mixture of an aqueous solution, slurry or gel, one or more metal salts with high water solubility can be used. For example, metal nitrate, metal sulfate, metal chloride, metal acetate, and metal formate, etc., can be used. Organic solvents, such as alcohols, isopropanol, etc., can be used to dissolve and/or disperse metal-containing precursors with low water solubility. In some cases, the pH value of the liquid mixture can be adjusted to increase the solubility of the one or more precursor compounds. Optionally, chemical additives, gelation agents, and surfactants, such as ammonia, EDTA, etc., can be added into the liquid mixture to help dissolve or disperse the precursor compounds in a chosen solvent. The mist of the liquid mixture may be generated by a mist generator, such as a nozzle, a sprayer, an atomizer, or any other mist generators. Most mist generators employ air pressure or other means to covert a liquid mixture into liquid droplets. The mist generator can be coupled to a portion of the drying chamber to generate a mist (e.g., a large collection of small size droplets) of the liquid mixture directly within the drying chamber. As an example, an atomizer can be attached to a portion of the drying chamber to spray or inject the liquid mixture into a mist containing small sized droplets directly inside the drying chamber. In general, a mist generator that generates a mist of mono-sized droplets is desirable. Alternatively, a mist can be generated outside the drying chamber and delivered into the drying chamber. Desired liquid droplet sizes can be adjusted by adjusting the sizes of liquid delivery/injection channels within the mist generator. Droplet size ranging from a few nanometers to a few hundreds of micrometers can be generated. Suitable droplet sizes can be adjusted according to the choice of the mist generator used, the precursor compounds, the temperature of the drying chamber, the flow rate of the first gas, and the residence time inside the drying chamber. As an example, a mist with liquid droplet sizes between one tenth of a micron and one millimeter is generated inside the drying chamber. At step 104, the mist of the liquid mixture is dried (e.g., removing its moisture, liquid, etc.) at a drying temperature for a desired first residence time and form into a first gas-solid mixture with the flow of the first gas within the drying chamber. As the removal of the moisture from the mist of the precursor compounds is performed within the drying chamber filled with the first gas, a first gas-solid mixture composing of the heated first gas and the precursor compounds is formed. Accordingly, one embodiment of the invention provides that the first gas flowed within the drying chamber is used as the gas source for forming a first gas-solid mixture within the drying chamber. In another embodiment, the first gas flowed within the drying chamber is heated and the thermal energy of the heated first gas is served as the energy source for carrying out drying reaction, evaporation, dehydration, and/or other reactions inside the drying chamber. The first gas can be heated to a temperature of between 70° C. to 600° C. by passing through a suitable heating mechanism, such as electricity powered heater, fuel-burning heater, etc. In one configuration, the first gas is pre-heated to a temperature of between 70° C. to 600° C. prior to flowing into the drying chamber. Optionally, drying the mist can be carried out by heating the drying chamber directly, such as heating the chamber body of the drying chamber. For example, the drying chamber can be a wall-heated furnace to maintain the drying temperature within internal plenum of the drying chamber. The advantages of using heated gas are fast heat transfer, high temperature uniformity, and easy to scale up, among others. The drying chambers may be any chambers, furnaces with enclosed chamber body, such as a dome type ceramic drying chamber, a quartz chamber, a tube chamber, etc. Optionally, the chamber body is made of thermal insulation materials (e.g., ceramics, etc.) to prevent heat loss during drying. The first gas may be, for example, air, oxygen, carbon dioxide, nitrogen gas, hydrogen gas, inert gas, noble gas, and combinations thereof, among others. For example, heated air can be used as an inexpensive gas source and energy source for drying the mist. The choice of the first gas may be a gas that mix well with the mist of the precursors and dry the mist without reacting to the precursors. In some cases, the chemicals in the droplets/mist may react to the first gas and/or to each other to certain extent during drying, depending on the drying temperature and the chemical composition of the precursors. In addition, the residence time of the mist of thoroughly mixed precursor compounds within the drying chamber is adjustable and may be, for example, between one second and one hour, depending on the flow rate of the first gas, and the length and volume of the path that the mist has to flow through within the drying chamber. The mist of the liquid mixture is being dried within the drying chamber using the heated first gas flowed continuously and/or at adjustable, variable flow rates. At the same time, the dried solid particles of precursors are carried by the first gas, as a thoroughly-mixed gas-solid mixture, through a path within the drying chamber, and as more first gas is flowed in, the gas-solid mixture is delivered out of the drying chamber and continuously delivered to a first gas-solid separator connected to the drying chamber. Not wishing to be bound by theory, in the method 100 of manufacturing a battery material using one or more precursor compounds, it is contemplated that the one or more precursor compounds are prepared into a liquid mixture and then converted into droplets, each droplet will have the one or more precursors uniformly distributed. Then, the moisture of the liquid mixture is removed by passing the droplets through the drying chamber and the flow of the first gas is used to carry the mist within the drying chamber for a suitable residence time. It is further contemplated that the concentrations of the precursor compounds in a liquid mixture and the droplet sizes of the mist of the liquid mixture can be adjusted to control the chemical composition, particle sizes, and size distribution of final reaction product particles of the battery material. Next, at step 106, the first gas-solid mixture comprising of the first gas and the precursors mixed together are separated into a first type of solid particles and a first side product using, for example, a first gas-solid separator. The first type of the solid particles may include thoroughly-mixed solid particles of the precursors. Accordingly, a first stage of the method 100 of preparing a battery material include obtaining a first type of solid particles from a first gas-solid mixture comprised of a first gas and one or more precursor compounds. In the method 100 of preparing a final material product in multiple stages, it is contemplated to perform one or more reactions of the precursor compounds in a first drying stage, two or more reaction stages, one or more cooling stages, etc., in order to obtain final reaction products in desired size, morphology, and crystal structure, which are ready for further battery applications. Not wishing to be bound by theory, it is designed to perform the reaction of the precursors in two or more reaction stages to allow sufficient time and contact of the precursor compounds to each other, encourage nucleation of proper crystal structure and proper folding of particle morphology, incur lower-thermodynamic energy partial reaction pathways, ensure thorough reactions of all precursor compounds, and finalize complete reactions, among others. The first type of solid particles comprising the precursor compounds are then processed in two or more processing stages (e.g., a second processing stage and a third processing stage) using at least a first reaction module designed for initiating reactions, and a second reaction module designed for completing reactions and obtaining final reaction products. Additional reaction modules can also be used. In one embodiment, the first reaction module includes one or more gas-solid feeders for processing the first type of solid particles into one or more types of solid particles, where a portion of them are partially reacted (some complete reactions may occur). The second reaction module includes one or more reactors for processing the one or more types of solid particles into final reaction products to ensure complete reactions of all the reaction products. Accordingly, the method 100 may include a first processing stage of drying a mist of a liquid mixture and obtaining a first type of precursor-containing solid particles using a processing module comprised of a drying chamber and a first gas-solid separator. The method 100 may further include a second processing stage of reacting the first type of solid particles using a reaction module comprised of one or more gas-solid feeders and one or more gas-solid separators. At step 108, a flow of a second gas is delivered to the first type of solid particles once the first type of solid particles is separated from the first side product. For example, a first gas-solid feeder can be used to mix the second gas with the first type of solid particles collected from the first gas-solid separator and form a second gas-solid mixture inside the first gas-solid feeder. In one embodiment, the second gas is heated to a second temperature, which may be a desired reaction temperature, such as a temperature of between 400° C. to 1300° C., and flowed into the first gas-solid feeder to serve as the energy source for initiating one or more reactions to the precursor-containing first type of solid particles. In another embodiment, the second temperature within the first gas-solid feeder is a temperature higher than the first temperature used within the drying chamber. A Gas-solid feeder is used as a quick and easy in-line delivery mechanism to mix solids with gases and without the drawback of using a chamber reactor (e.g., a furnace), which often requires periodic maintenance and repair. Exemplary gas-solid feeders include, but are not limited to, a venture feeder, a rotary feeder, a screw feeder, a table feeder, a belt feeder, a vibrating feeder, a tube feeder, and combinations thereof, among others. At step 110, the second gas-solid mixture reacted at the second temperature is then separated into a second type of solid particles and a second side product using, for example, a second gas-solid separator. The second type of the solid particles may be contain a solid particle mixture comprising of unreacted, partially reacted, and/or compete reacted particles of the precursors. The second side product may include unwanted solvent molecules, reaction by-products, and/or waste gases. Optionally, the second processing stage of performing partial reactions of the precursor compounds may be conducted in series and/or in-line consecutively to obtain additional types of solid particles using additional reaction modules comprised of at least one gas-solid feeder and at least one gas-solid separator. For example, at step 112, the second type of solid particles may be delivered into, for example, a second gas-solid feeder, and a flow of a third gas may be flowed into the second gas-solid feeder to mix with the second type of solid particles and form a third gas-solid mixture. The flow of the third gas may be heated to a third temperature of between 400° C. to 1300° C., and to serve as the energy source to the third gas-solid mixture for one or more reactions. In one embodiment, the third temperature within the second gas-solid feeder is a temperature higher the second temperature used within the first gas-solid feeder. At step 114, the third gas-solid mixture reacted at the third temperature is then separated into a third type of solid particles and a third side product using, for example, a third gas-solid separator. The gas-solid mixtures within the first and the second gas-solid feeder may undergo one or more partial or complete reactions. Exemplary reactions of the various type of solid particles within the gas-solid feeders may include, any of oxidation, reduction, decomposition, combination reaction, phase-transformation, re-crystallization, single displacement reaction, double displacement reaction, combustion, isomerization, and combinations thereof, among others. For example, the second and third gas-solid mixture may be partially or completely oxidized, such as oxidizing the precursor compounds into an oxide material. Exemplary second and third gases include, but are not limited to air, oxygen, carbon dioxide, an oxidizing gas, hydrogen gas, a reducing agent, nitrogen gas, inert gas, noble gas, and combinations thereof, among others. For an oxidation reaction, an oxidizing gas, such as air, oxygen, etc., can be used. For a reduction reaction, a reducing gas, such as hydrogen gas, ammonium, etc., can be used as the second gas. In addition, nitrogen gas or inert gas can be used as carrier gas. As an example, heated air is used as the gas source at steps 108, 112. It is contemplated to remove unwanted side products at the steps 110, 112, 114, 116 of the second processing stage, such that various types of solid particles can be further processed without interference from the reaction side products. Accordingly, a second stage of the method 100 of preparing a battery material includes obtaining various types of solid particles (e.g., the second type and/or the third type of solid particles, which may be at least partially reacted) and removing unwanted side products. The method 100 may further include a third processing stage of reacting various type of solid particles into final reaction products using a reaction module comprised of at least one reactor, and optionally one or more gas-solid separators. At step 116, a flow of a fourth gas that is heated to a fourth temperature is flowed inside a reactor. Accordingly, a fourth gas-solid mixture containing the heated fourth gas and various types of solid particles delivered from the second and/or the third gas-solid separators is formed inside the reactor. In one embodiment, the fourth gas is heated to a desired reaction temperature, such as a temperature of between 400° C. to 1300° C., and flowed into the reactor
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