In this work, we have benchmarked different density functionals against accurate correlated WF methods for lithium-mediated electrolyte decomposition. In addition, we have compared the agreement between different correlated methods. Overall, DLPNO-CCSD(T) shows good agreement with canonical CCSD(T) for the reaction energies: when the reactions involving transition state is excluded, the MAD is 0.64 kcal/mol. Moreover, AFQMC also shows good agreement with a MAD of 1.74 kcal/mol and an average statistical uncertainty of ±1 kcal/mol. Considering the accuracy and cost of different correlated methods, we conclude that DLPNO-CCSD(T) is an excellent benchmark method for reaction energies of complex systems relevant to battery electrochemistry, i.e., reactions involving organic molecules and Li atoms. For barrier heights, it would likely help to improve the quality of the DLPNO-CCSD(T) calculations via tightened parameters (TCutPNO, TCutMKN, TCutPairs) and usage of the improved iterative "(T1)" correction. A performance comparison of all the density functional approximations used in this study reveals that CAM-B3LYP-D3, M06-2X-D3, and double-hybrids perform very well for this system. Perhaps most importantly, the DFT transition state barrier heights vary from 3.00 to 17.15 kcal/mol, when the CCSD(T) barrier height is 12.84 kcal/mol. GGAs in particular are especially prone to significant underestimation of the barrier height by up to 10 kcal/mol. Therefore, we urge caution when interpreting the results of GGA-based molecular dynamics simulations of electrolyte decomposition and SEI formation.However, despite the good performance of meta-GGAs, range-separated hybrids, and double-hybrids observed in this molecular study, we note that these families of functionals are limited in their applicability to periodic metals. The development of accurate and broadly applicable electronic structure methods for surface chemistry is an important task for the community, especially towards the development of accurate force fields for reactive molecular dynamics simulations.