Enhanced Cycleability of LiNi0.5Mn1.5O4 Cathode with Zn Substitution

Introduction

In the last decade, lithium-ion batteries are expected to be applied in electric vehicles. High voltage or capacity cathode materials are required to achieve high energy batteries. Spinel LiNi_0.5Mn_1.5O₄ draws much attention owning to a voltage plateau as high as 4.7 V vs. Li/Li⁺, accepted capacity, excellent rate capability and good safety.^[1] However, high voltage plateau brings side reactions between the cathode and the electrolyte at high voltage which cause capacity fade of spinel LiNi_0.5Mn_1.5O₄.^[2] Partial substitution of Mn or Ni by metal elements such as Al, Cr, Fe, Co etc.^[3] has been proven to be successful to enhance its cycleability. Series substitutions of Mg instead of Ni are achieved to modify the spinel. The cycleability has been much improved by Mg substitution even charged to 5.1 V. Experimental

LiZn_xNi_0.5-xMn_1.5O₄ was prepared via a sol–gel method assisted by citric acid as chelating agents. Citric acid was firstly dissolved in distilled water. Stoichiometric LiNO₃, Zn(NO₃)₂•6H₂O， Ni(NO₃)₂•6H₂O and Mn(NO₃)₂•4H₂O were added to the solution above with continuous stirring. The final solution was evaporated at 80 ºC until a gel was obtained and then dried at 140 ºC for 10 h. The powders was preheated at 400 ºC for 10 h and then calcined at 850 ºC for 24 h in air. The structure and morphology of the samples were studied by X-ray diffraction (XRD). Electrochemical measurements were carried out with CR2032 coin cells. The obtained materials were mixed with C-black and PVDF (80:10:10 wt %) in N-methyl pyrrolidinone solvent. After coating the above slurries on Al foils, the electrodes were dried at 80 ºC in vacuum. In the coin-cell tests, metallic lithium foil was used as the counter and reference electrodes; the electrolyte was 1 M LiPF₆in a 1:1 solvent mixture of ethylene carbon (EC)/dimethyl carbonate (DMC). Results and Discussion

Figure 1 displays XRD patterns for LiZn_xNi_0.5-xMn_1.5O₄ (x=0, 0.02, 0.04, 0.08, 0.25, 0.5) powders. The powders all show cubic structure. Rocksalt impurity phase Ni_1-xO can be found in the samples of x=0 and 0.02. Impurity Ni_1-xO, Li_1-xNi_xO, or Ni₆MnO₈ as a well-known secondary phase in samples of LiZn_xNi_0.5-xMn_1.5O₄ can be observed at around 37.6˚ and 45.7˚^[1–3] when x=0 and 0.02. The impurity intensity decreases with increasing Zn content and the x=0.04 and 0.08 samples show pure spinel phase. While x=0.25 and 0.5, a new impurity phase ZnMn₂O₄ emerges. In spinel structure, Zn ions prefer occupying tetrahedral sites (8a), such as ZnFe₂O₄ and ZnAl₂O₄. As a result, Zn ions  are supposed to exist in tetrahedral sites (8a) instead of Li ions.

Cycling performance for the electrodes made of LiZn_xNi_0.5-xMn_1.5O₄ (x=0, 0.02, 0.04, 0.08) at 0.5 C between 3.5–4.9 V is shown in Figure 2. The capacity mainly origin from Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ redox couples for the spinel LiNi_0.5Mn_1.5O₄. As a result, the capacity decreased with increasing Zn substitution content instead of Ni. However, the cycleability has been much improved by Zn substitution especially when x = 0.08. Only 65 % capacity retention was obtained for the bare electrode. Compared to the pristine electrode, the LiZn_0.08Ni_0.42Mn_1.5O₄ electrode delivers 98 mAh/g for the first cycle with remarkable capacity retention of 95 % after 100 cycles. XPS data reveals that a Li-permeable SEI layer formed on the surface of the electrode which protects the electrolyte from oxidation at high voltage^[4].

In conclusion, LiZn_xNi_0.5-xMn_1.5O₄ spinels (x=0, 0.02, 0.04, 0.08, 0.25, 0.5) have been successfully prepared by sol-gel method. Zn substitution significantly improved the cycleability of the spinels. A plausible explanation is that a Li-permeable SEI layer formed on the surface of the Zn substitution electrode which protects the electrolyte from oxidation at high voltage.