Overcoming the ultra-high C-rate limitation of LiFeBO3 with careful optimization of carbothermal synthesis conditions

Abstract

Carbon coating with sucrose as the precursor under a moderate synthesis temperature of 500 °C results in a high-purity monoclinic LiFeBO3 phase. This material has a high specific discharge capacity of 200 mAh/g at 0.02 C, which is close to its theoretical capacity. However, its poor performance at high C-rates limits its utility in electric vehicles. Achieving ultra-high C-rate capability of more than 5 C for LiFeBO3 cathode for lithium-ion batteries is uncommon. To overcome this challenge, this study carefully optimizes the synthesis conditions, such as temperature and reducing atmosphere. In this work we utilize carbothermal synthesis method for modulating the reducing conditions. Usually, the carbothermal reaction is carried out using carbon at high temperatures which leads to the formation of CO and CO2 gases. In our work, we have utilized a mixture of sucrose and oxalic acid as the carbon source, that is intimately mixed with the other precursors to create moderate reducing conditions. Oxalic acid decomposes to generate CO which helps to attain the limited reducing atmosphere condition necessary to retain the low valence state of iron. This is important because the low valence state of iron is essential for a good rate-performance. However, if the amount of carbon source is too low, it can lead to poor rate-performance. On the other hand, if the amount of carbon source is too high, it can generate impurity phases, which can be detrimental to the material’s properties. The optimized sample can deliver electrochemical activity at ultra-high C-rates, even up to 30 C, which is achieved for the first time in this material. Comprehensive spectroscopic analyses reveal that multitude of factors, including improved particle crystallinity and better graphitization, favor the retention of the monoclinic phase during electrochemical cycling. A stable specific discharge capacity of 104 mAh/g at 10 C sets it as a benchmark for this material.