Saturated lithium oxalate solution for surface optimization of Ni-rich cathodes during water-washing processes

Abstract

Water-washing is routinely employed in industry to remove the residual lithium compounds (RLCs) from the solid-state synthesized Ni-rich layered cathodes. However, these processes remain controversial, including potential damage to the surface structure with phase transformation and then reduce specific capacity. In this work, saturated lithium oxalate solution is used as an alternative to deionized water for treating the LiNi_0.8Co_0.1Mn_0.1O₂ (NCM) cathodes. Compared to traditional washing strategies, samples exhibit significantly improved electrochemical performances. The saturated lithium oxalate solution washed samples (NCM-LC) achieve a significantly improved discharge capacity of 211.2 mAh g⁻¹ at 0.1C with an initial Coulombic efficiency (ICE) of 89.99 %, and present a good capacity retention of 84.47 % after 500 cycles at 3–4.3 V. By contrast the water washed samples (NCMW) present reduced initial discharge capacity of 182.5 mAh g⁻¹ and then rapidly decay to 110.55 mAh g⁻¹ after 200 cycles, the capacity retention is only 62.35 %. It is found that the saturated lithium oxalate suppresses the formation of NiOOH compounds on the sample surface, thus inhibiting the formation of NiO rock-salt phase. Furthermore, a protective coating can be formed on the sample surface after these treatments, which further improving the surface chemical stability and structural integrity of samples. This work offers a promising strategy for removing RLCs, which exert minimal impact on the electrochemical performances of Ni-rich layered cathodes.