Reactivity of LiNi0.5Mn1.5O4 in (Acidic) Water and Impact on the Electrochemical Performance

Abstract

As the world-wide demand for Li-ion batteries (LIBs) is continuously rising, the efforts to make the production of LIBs more sustainable are rapidly increasing as well.[1] Given that aqueous processing has already become the standard for the negative electrode, adapting also the processing of the positive electrode to water-based fabrication routes would eventually allow for completely eliminating the use of harmful N-methyl-2-pyrrolidone (NMP). In addition to the beneficial impact on sustainability and environmental friendliness, the use of water would enable avoiding the need for the complex NMP recovery and, thus, also result in lower costs for the electrode manufacturing.[2] However, the contact of commonly used oxide-type cathode materials with water leads to undesired side reactions, where amongst others LiOH and Li2CO3 form on the particle surface. Previous studies have shown that the immediate negative impact of LiOH, i.e., the rise of the slurry pH level, can be mitigated by the addition of phosphoric acid during processing to prevent the corrosion of the Al current collector.[3 , 4] While this strategy has proven viable for enabling the aqueous electrode manufacturing of all kinds of Li-ion positive electrode materials,[5] we are herein elucidating the direct impact of aqueous and (phosphoric) acidic treatments on the high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) active material itself prior to the slurry preparation. Our extensive structural investigations using a comprehensive set of complementary techniques revealed that the water and acid treatment did not evoke any changes in the bulk material. On the material surface, however, a variety of different species is formed – some with a beneficial effect on the electrochemical performance (e.g., Li3PO4), some with a detrimental impact (e.g., Li2CO3 and transition metal carbonates). Nevertheless, the careful adjustment of the (acidic) aqueous treatment eventually results in a capacity retention of 95% after 500 cycles – clearly outperforming the NMP-processed reference.