Full length articleStudies on electrochemical reversibility of lithium tungstate coated Ni-rich LiNi0.8Co0.1Mn0.1O2 cathode material under high cut-off voltage cycling
Introduction
Compared to these commercialized materials such as layered LiCoO2 (140 mAh g−1), spinel LiMn2O4 (120 mAh g−1) and olivine LiFePO4 (160 mAh g−1) [[1], [2], [3], [4]], the Ni-rich LiNixM1−xO2 (0.6 < x < 1; M = Co, Al, Mn, etc.) material might be the most preferable layered cathode material for high-energy lithium ion batteries (LIBs) owing to the advantages of high practical specific energy (excess 720 Whk g−1 originated from the high practical reversible capacity of above 200 mAh g−1 and a high discharge voltage of 3.7 V), high range of reversible Li intercalation/de-intercalation, and environment friendly [[5], [6], [7]]. Recently, research has been extensively devoted to improving the energy density of the Ni-rich cathode materials in order to further meet the needs in electric vehicles, including increasing the Ni content (major redox pairs) and the charge cut-off voltage. Among them, more attention has been paid to LiNi0.8Co0.1Mn0.1O2 as a representative Ni-rich cathode material by virtue of relatively low cost, reversible discharge capacity (>200 mAh g−1) and high energy density [[8], [9], [10]]. Although the Ni-rich cathode material is normally charged to 4.3 V, its charge cut-off potential can be increased to a higher voltage (4.6 V) in order to obtain more capacity. However, the severe capacity degradation is aggravated because of the undesirable interfacial reactions between electrode materials and electrolytes during high cut-off voltage cycling [[11], [12], [13]]. For example, Ni4+ formed by the interfacial reactions between highly delithiated LiNixM1−xO2 and electrolyte tends to react with the organic electrolyte aggressively and yield undesirable side reaction products leading to the increasing resistance of the cathode electrolyte interphase film (CEI film) [14]. In addition, the structural instability of nickel-rich layered cathode material is one of the key problems for its long-term storage, because some lithium precipitating from the bulk and reacting with carbon species may form LiOH/Li2CO3 and other impurities during long-term storage [15]. Moreover, these lithium impurities may accelerate the decomposition of electrolyte and generate more HF to erode the active materials [16,17]. Therefore, the nickel-rich layered cathode is limited in practical commercial applications due to its remarkable degradation of the cycling stability and rate capability during high cut-off voltage cycling [14].
Substantial investigation has been proved that surface modification is an effective strategy to defend the cathode against aforementioned problems throughout the whole applied voltage window [18,19]. Different coating materials including metal oxides [20,21], phosphates [22], fluorides [23], sulfides [24] and carbon [25] have been reported as good coating materials that can effectively insulate the active material from the electrolyte and suppress side reactions, which improve the long-term cyclic performance [26,27]. Nevertheless, these surface modification materials are detrimental to the rate performance and even reversible capacity of the LiNi0.8Co0.1Mn0.1O2 cathode materials as a result of their poor ionic and electronic conductivity [28]. In light of the above issues, Lithium tungsten oxides (Li2WO4), as a kind of fast Li-ion conductor, may be the one of best coating materials because it can improve the lithium-ion diffusion and reduce the interfacial resistance [29,30]. Aida et al. [31] found the stability and rate capability of Li-rich Li1+xNi0.35Co0.35Mn0.30O2 could be highly enhanced by ammonium tungstate modification. Fu et al. [14] reported that electrolyte solvent decomposition may be suppressed by Li2WO4 modification. Encouraged by the results, we focused on the Ni-rich LiNi0.8Co0.1Mn0.1O2 material, and selected Li2WO4 as coating layer to study its electrochemical reversibility at high cut-off voltage (4.6 V) and long term storage.
In this work, no impurities are produced in the synthesis of the tungstate-modified LiNi0.8Co0.1Mn0.1O2 electrode by a one-step synthesis followed by high temperature calcination method, because only easy vaporizable products (NH3 and H2O) are generated in the neutralization reaction given below [31].
Besides, As W has larger atomic weight, higher chemical valence and larger atomic radius, the W element possibly prefers to dispersing on the surface and the grain boundary of the primary particles [14]. The cyclic performance of LiNi0.8Mn0.1Co0.1O2 cathode material with Li2WO4 coating is systematically investigated under high cut-off voltage (4.6 V) and long term storage. With the inhibition of side reaction and facilitation of lithium diffusion, the cycle stability of the LiNi0.8Mn0.1Co0.1O2 material is obviously improved after 2 wt% Li2WO4 coating. Simultaneously, the problems of polarization and voltage reduction of LiNi0.8Mn0.1Co0.1O2 are also alleviated. Our results show that Li2WO4-surface modification is of great significance to improve the cycle stability and reduce polarization of Ni-rich cathode materials under high cut-off voltage (4.6 V).
Section snippets
Synthesis of pristine and modified samples
The Ni0.8Co0.1Mn0.1(OH)2 precursor was prepared by the co-precipitation method [32]. First, 2.0 mol L−1 metal solution was formed via NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O (cationic ratio of Ni: Co: Mn = 8:1:1). 4.0 mol L−1NaOH solution as a precipitating agent and 2.0 mol L−1 NH3·H2O solution as a chelating agent were prepared in the same way. The sulfate salt solution, NaOH solution and NH3·H2O solution were separately slowly dripped into a reactor under the atmosphere of pure nitrogen. The
Morphology characterization of cathode materials
In order to research the crystal structure of the NCM and Li2WO4 coated samples, XRD detection method was carried out. As shown in Fig. 2a, the peaks of all samples can be indexed to a well-defined hexagonal R-NaFeO2 structure belonging to the R3̅m space group (PDF#09-0063), implying that the coating process has no significant effect on the crystal structure of the host material. The apparent splitting of (006)/(102) and (108)/(110) peaks demonstrate that the samples have good crystallinity [33
Conclusions
In summary, Lithium tungstate coating LiNi0.8Co0.1Mn0.1O2 cathode materials have been successfully prepared via a one-step synthesis and subsequent high temperature calcination method. It can be illustrated from the results of XRD, SEM, TEM, mapping and XPS that lithium tungstate coating layer can not only mainly remain on the surface of LiNi0.8Co0.1Mn0.1O2, but also make no difference to crystal structure, overall morphology and chemical state of the surface of host material. Benefited from
Acknowledgements
This work was supported by National Natural Science Foundation of China (Grant No. 21276286 and No. 21476268).
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