Structural evolution of bias sputtered LiNi_0.5Mn_1.5O₄ thin film cathodes for lithium ion batteries

Highlights

• LiNi_0.5Mn_1.5O₄ thin films have been deposited on stainless steel substrates.

• Various negative biases were applied on the substrates during deposition.

• The particle sizes of LNMO thin films decrease with increasing negative bias.

Abstract

LiNi_0.5Mn_1.5O₄ (LNMO) thin films have been deposited on stainless steel substrates using radio frequency (f = 13.56 MHz) magnetron sputtering, followed by thermal annealing in ambient atmosphere. Various negative biases were applied on the substrates during deposition. The structural evolution of LNMO thin films under different negative biases has been investigated and characterized by X-ray diffraction. All of the deposited films exhibit a crystalline spinel structure with a space group of Fd-3m, which is a so-called disordered phase. The results also indicate that particle size decreases with increasing negative bias. The electrochemical properties of the LNMO thin films as cathode materials for lithium ion batteries were investigated. Two distinctive voltage plateaus at ~ 4.7 V and at ~ 4.0 V (vs. Li⁺/Li) can be observed in the discharge curves, corresponding to the reactions of the disordered phase. The capacity of LNMO thin film electrodes under suitable negative bias can be optimized.

Introduction

Lithium ion batteries have been widely used as energy sources for portable electronic devices in the past few decades. Recently, hybrid electric vehicles, power tools, and other multi-functional devices have demanded lithium ion batteries with a high power density [1]. In this regard, high operating voltage cathode materials have been extensively studied.

The spinel LiMn₂O₄ has great promise for future use in lithium ion batteries because of its low cost, high abundance, environment friendliness, and safety [2]. LiMn₂O₄ also possesses a relatively high voltage of ~ 4.0 V, or over 4.5 V (vs. Li⁺/Li), after transition metal substitution, which makes it more suitable for high power applications than other cathode materials [3]. However, LiMn₂O₄ has limitations such as Mn^3 + dissolution at high temperatures (> 55 °C) and Jahn–Teller distortion at low voltage (2.8 V), causing structure instability and leading to poor cycling stability [2], [4]. Therefore, cathode materials with high voltage and high stability are desirable. In this regard, 3d-transition metal (M) substitution of LiMn₂O₄ has been previously studied in the field [5]. The LiM_xMn_1 − xO₄ cathode materials can provide a high operating voltage of ~ 5 V vs. Li⁺/Li through the redox reaction of M^3 + to M^4 + for M is Cr or Co, and M^2 + to M^4 + for M = Ni [6]. In addition, Ni substitution can restrict the Mn ion dissolution by maintaining the Mn at valence state of 4 + [7]. Spinel LiNi_0.5Mn_1.5O₄ (LNMO)¹ is a promising candidate because the Ni substitution not only reduces the Mn^3 + dissolution [8], [9], it also elevates the operating voltage (4.7 V vs. Li⁺/Li) [10], [11]. The high operating voltage may lead to the decomposition of electrolyte and that is known to induce the formation of a passive layer on the surface of the LNMO, causing the decay of the cycle life [12]. Therefore, various protective coatings on the surface of the LNMO have been studied [13].

Spinel LiMn₂O₄ is cubic with a space group of Fd-3m, where the Li atoms occupy the 8a site, Mn atoms the 16d site, and O atoms the 32e sites [14]. After Ni substitution, the structure of the LNMO is no longer that of a typical spinel. In the ordered phase of the LNMO, the Li atoms occupy 8c sites, Ni atoms and Mn atoms locate at the 4a and 12d sites, and O atoms inhabit the 8c and 24e sites (space group of P4₃32) [15]. When oxygen deficiency exists in the LNMO structure, impurity phases, such as Li_xNi_1 − xO and NiO, may occur [3], [16]. These impurity phases lead to the reduction of Mn^4 + to Mn^3 +. In order to maintain the valence state in the LNMO compound, some of the grains may crystallize into a so-called disordered phase [17], [18], where Li atoms locate at the 8a sites, Mn and Ni atoms are randomly distributed at the 16d sites, and O atoms occupy the 32e sites (space group of Fd-3m) [15].

Li–Ni–Mn–O related thin films have been fabricated by sputter deposition to study the influences of different Li⁺ stoichiometries [12]. The pure LNMO thin films without conducting additives and binders were found to be useful for scientific investigation. In the present study, the LNMO thin films were prepared by radio frequency (RF)² magnetron sputtering deposition and then characterized [12], [13]. Analysis of the structural evolution and electrochemical properties of the LNMO films deposited under different substrate biases were performed. This study demonstrates that the bias applied onto the substrate can effectively modify the structure of the film resulting in different electrochemical characteristic toward lithium. The cycling tests of LiNi_0.5Mn_1.5O₄ cells were directly affected by the electrolyte decomposition at high voltage (> 4.5 V) [13]. Therefore, this study is only focused on the relationship between the sputtering conditions, structural evolution, and related capacity within three cycles.