Synthesis And Modification Of Spinel LiNi_0.5Mn_1.5O₄ As Cathode Material For 5V Lithium-ion Battery

Spinel Li Ni0.5Mn1.5O4(LNMO) is regarded as a promising cathode material because of its high energy(~ 658 Wh kg-1) and high operating voltage(~ 4.7 V vs. Li/Li+) and fast threedimensional lithium-ion diffusion in the spinel lattices. Recent studies have shown that the non-stoichiometric spinel(Fd3m) with a certain amount of Mn3+ exhibits better electrochemical performance than that of the stoichiometric one(P4332) due to its higher electronic conductivity. Based on oxalate co-precipitation method, high-purity spinel LNMO cathode material with different amounts of Mn3+ was synthesized through a modified oxalate co-precipitation method by precisely tuning the p H value of the precursor solution and introducing appropriate excessive lithium source during preparation. Due to the low ionic conductivity of pure LNMO, we attempted to explore the synthetic process of spinel LNMO mixed with Li7La3Zr2O12(LLZO) based on the successful synthesis of high performance LNMO. We tried to use the high ionic conductivity(~ 10-4 S cm-1) of fast ion conductor LLZO to improve the electrochemical performance of composite material.CH3COOLi·2H2O、Ni(CH3COO)2·4H2O、Mn(CH3COO)2·4H2O were used as starting materials, C2H2O4·2H2O was used as precipitant. We studied the influence of different p H value of the precursor resolution on the performance of battery. With the increase of p H value, the morphology became more regular, crystallization degree became higher, and besides, the particle size got a bit larger through SEM analysis. Apart from LNMO phase, we still found a very weak Lix Ni1-x O impurity. And the diffraction peaks shifted to a lower angle when deviating from the p H value of 8 through XRD analysis, implying an enlargement of the cubic lattice parameters. In order to eliminate the Lix Ni1-x O impurity, we added excessive Li source and studied the effect of different amounts of Li source on the performance of battery. Through XRD analysis, the impurity disappeared with the increase of lithium source. However, impurity phase appeared again with 10% excessive Li salt. Comparing multi-group experiments, we got optimized experimental conditions, that was to say the p H value of the precursor resolution was regulated as 8 and the Li source was 5% excess. LNMO material could retain 106 m Ah g-1 and the reversible capacity was retrieved 140 m Ah g-1 when the rate was back to 0.1 C. When cycled at 0.2 C rate, the average capacity was 136 m Ah g-1, higher than the sample with stoichiometric Li source(~ 128 m Ah g-1).Upon the basis of the experiments above, we attempted to synthesize spinel LNMO cathode material mixed with fast ion conductor LLZO. Thus we studied the effect of the amount of LLZO under 5% and 10% excessive Li salt respectively. With 5% excessive Li salt, the optimized mass ratio of LNMO to LLZO was 80:1. The specific discharge capacity was 142 m Ah g-1、135 m Ah g-1、135 m Ah g-1、134 m Ah g-1、130 m Ah g-1、117 m Ah g-1、90 m Ah g-1 when at 0.1 C、0.2 C、0.5 C、1 C、2 C、5 C、10 C, respectively. Composite material delivered an average capacity of 131 m Ah g-1 when cycled at 1 C and retained 99.7%of the initial capacity after 50 cycles. However, with 10% excessive Li salt, the sample with the mass ratio of 80:1 had a large capacity fade, arised from the increase of residual lithium on the material surface according to our suppose. Therefore, we tried to mix the composite material with PVDF to eliminate the residual lithium on the material surface. After sintering, we were very pleased to find the capacity retention was improved by 6%.