2 ) compared to the current graphite anode (372 mA-h/g 3 ). Furthermore, silicon is the secondmost abundant element in the earth's crust, making it a readily available material. However, its practical implementation is limited by a main bottleneck, the huge volumetric expansion after lithiation (310% for Li 22 Si 5 ) which causes a large build-up of stress, resulting in the pulverization of the material and immediate capacity loss during cycling. An example is the loss of 70% of the capacity of silicon anode made from 10 µm silicon powder within the first five cycles. There has been a surge in research output to tackle this bottleneck over the past decade. Specifically, one focus of research is on using micro-nano sized particle silicon anodes as they have been shown to resist cracking and accommodate the volumetric change. However, the improvement is still not sufficient for practical application. Other cutting edge research concentrates on modifying the anode by the formation of alloys, composites, core-shell structures, 20,21 films and porous systems. On the fundamental aspect, unavoidable volume expansion in each crystalline grain of silicon upon lithiation creates important mechanical stress in the material, responsible for rupture. A major mechanism to relieve this stress would be to allow facile sliding at the grain boundaries. Previous experimental observations of GB sliding have been predominantly limited to metallic materials such as Al, Cu, Sn, Zn, and Mg. To our knowledge, there has been no work in the literature for GB sliding in silicon. This comes from the fact that silicon has strong covalent bonds, which may render GB sliding highly activated and inoperative at near ambient temperature. In this manuscript, we study GB sliding in silicon using first-principle atomistic simulations. We show that GB sliding in silicon is activated and that doping silicon with aluminum markedly facilitates GB sliding, and is, therefore, a potential solution to improve the mechanical properties and durability of silicon anodes upon cycling. We show that the small amount of aluminum segregates in the grain boundaries (GBs) of silicon and greatly facilitates GB sliding. The prevalence of the Σ3 {111} GB in polycrystalline materials has been well-documented in numerous previous studies and is revealed in our GB characteristics quantification by electron backscattered diffraction (Supporting Information Figure ). Recognizing its significance, we have consistently employed the Σ3 {111} GB in all our simulations. We have devised a unique model to perform grain boundary sliding simulations. Our investigative approach encompasses the utilization of basin hopping, a global optimization technique to understand the segregation of aluminum in the GB. Through this computational framework, we seek to gain insights into the influence of aluminum on GB sliding behavior, hence enabling the reorganization of the polycrystalline silicon anode without mechanical failure during lithiation. To validate the simulation results, micro-sized polycrystalline silicon is experimentally doped with 5 wt.% (4.9 mol.%) aluminum and mixed by high-energy ball milling. Using our charge-discharge cycling test of half cells, we show that aluminum-doped silicon anode exhibits improved capacity retention than the undoped counterpart. We believe that this is an innovative and cost-effective way of improving the cyclic stability of silicon anodes.