Electrochemical performance of high voltage LiNi0.5Mn1.5O4 based on environmentally friendly binders
Introduction
Road transport accounts for approximately 16% of total global carbon dioxide (CO2) emissions [1]. Electrical vehicles (EVs) have higher energy efficiency and lower CO2 emissions in comparison with a standard internal combustion engine (ICE) vehicle [2]. Therefore, the widespread diffusion of EVs should be supported by worldwide government policies [3,4]. Norway announced that it will ban petrol-powered cars by 2025, Germany and India by 2030, and France and the U.K. by 2040 [5]. Although, in recent years, EV sales increased [6], the widespread EVs market penetration should be fostered to achieve the expected CO2 emission reduction. The development of lithium-ion batteries (LIBs) that are safer, more environmentally friendly, less costly, and with an energy density higher than 250 Wh/kg with respect to the current LIBs is crucial for the green energy transition.
LiNi0.5Mn1.5O4 (LNMO) is a key cathode material for the development of next-generation environmentally friendly LIBs thanks to the absence of cobalt in its crystal structure, low cost, high-operating potential (4.7–4.75 V vs. Li+/Li) that enables high energy density [7]. High-voltage spinel oxide materials could replace the widespread commercial layered nickel cobalt oxides in several applications [8]. According to the European Strategic Research Agenda for Batteries, lithium batteries are classified into five generations depending on cell chemistry [9]. LIB cells featuring LNMO cathode are named generation 3b, and the market deployment is foreseen around 2025 (after NMC622-NMC811).
The electrode binder also plays a critical role in the electrode performance, albeit its relatively low content of a few percent (2 to 10 wt%) with respect to the total electrode composition [10]. Indeed, the binder does not only glue the active materials and conductive additives together, but also strongly influences electrode processing and electrochemical performance [6,7,11], [[12], [13], [14], [15], [16]]. Poly (vinylidene fluoride) (PVdF) is the most extensively used binder for positive and negative electrode materials. However, PVdF is only soluble in limited polar solvents and commonly N-methyl-2- pyrrolidone (NMP) is used to prepare the slurry [17]. NMP is not an ideal solvent since it is expensive, volatile, and toxic [2,7,11,12,18]. Efforts toward reducing battery cost and environmental impact have transitioned from a nonaqueous-based to aqueous-based electrode slurry processing and solvent recycling [19]. Cost benefits are quite obvious: NMP (1–3 $ kg−1) and PVdF (8–10 $ kg−1) are rather expensive when compared to water (0.015 $ L−1), water-soluble binders like sodium carboxymethyl cellulose (CMC) (2–5 $ kg−1) [20,21], and polysaccharide guar gum (GG) (e.g. in Indian market 0.5–1$/kg).
Numerous studies investigated water-soluble binders for lithium cathodes [6]. Pieczonka et al. studied lithium polyacrylate (LiPAA) water-based binder for LNMO electrodes, demonstrating better adhesion and cycle performance than PVdF-based ones [22]. The CMC water-soluble polymer was successfully proposed as a binder for LNMO electrodes thanks to the improved distribution homogeneity of conductive additive and binder mixture with respect to PVdF, thus enhancing the electronic conductivity [23].
GG is a polysaccharide derived from the seeds of Cyamopsis tetragonolobus and, as a safe compound, is also used as a food thickener [13,24]. The GG powder is also widely used as a stabilizer, thickener, and suspending agent in construction, textile, cosmetics, pharmaceuticals, paper, oil well drilling, and mining industries. While CMC consists of a linear chain of glucose backbone and it is a cellulose derivative containing trans-hydroxyl pairs in its glucose component, GG has cis-hydroxyl pairs and branched galactose grafts [13]. GG and CMC binders are cheap and green binders for next-generation batteries.
In particular, GG was used for the first time as an aqueous binder in LIB cells to prepare layered lithium-rich oxide (LLRO) cathodes [24], demonstrating that 10 wt% GG protects against side reactions and electrode corrosion. Lu et al. proposed the use of GG in LiS batteries [25], and Carvalho et al. presented 3 wt% and 5 wt% GG binder in NMC cells [2]. Recently, Kuenzel et al., for the first time, used GG binder for LNMO//graphite full-cell and demonstrated remarkably high cycling stability with 80% capacity retention after 1000 cycles at 1C at room temperature [26].
The rapid capacity fade of LNMO at elevated temperatures is a well-known phenomenon [27,28]. The electrochemical performance of LNMO dramatically decays during prolonged charge/discharge cycling at high C-rates or cycling at elevated temperatures [[29], [30], [31]]. The presence of parasitic side reactions under high LNMO operating potential (~ 4.7 V), which are further promoted at high temperatures (50–60 °C), was highlighted as the leading cause of poor cycle life [32,33]. However, the LIBs must survive wide temperature service under harsh conditions. Therefore, to obtain reliable data on cells' real durability, a cycling test at elevated temperatures and high C-rates is mandatory. Such a key point is addressed in the present work, where the cycle numbers are extended up to 1000.
The present work primarily focuses on cycling performance at elevated temperature (50 °C) and relatively high C-rates (1C and 3C) of LNMO-based electrodes featuring GG and CMC binders at different amounts compared to those of electrodes with PVdF binder. Since a lower binder amount increases the volumetric energy density of the cell, we also investigated the use of 2 and 1 wt% of the GG binder. Operando differential electrochemical mass spectroscopy (DEMS) analysis was also carried out to link the capacity fade of LNMO electrodes with different binders to the evolution of gaseous or volatile products of electrochemical reactions occurring over cycling inside the half-cells.
Section snippets
Experimental
Commercial pristine LNMO powder MTI™ was used as a cathode. In addition, V-doped LNMO (LiNi0.4V0.1Mn1.5O4) was prepared using a sol-gel method described in our previous work [34]. The V-doped cathode is also used as a benchmark for the tests at 50 °C since it displays long-lasting cycling performance. V-doped LNMO powders were synthesized at 900 °C for 10 h in a flowing air atmosphere (0.35 Standard Liter Per Minute- SLPM /SIARGO™) with a heating rate of 2 °C min−1 and a cooling rate of 3 °C min
Structural characterization
All diffraction peaks of LNMO-based powders in Fig. 1 can be attributed to the Fd-3 m (JCPDS no. 80–2162) space group, as reported in our earlier work [34]. It is worth noting that the disordered cubic Fd-3 m phase shows better performance than the ordered P4332 one due to its higher electronic conductivity, rate capability, and lithium-ion diffusion coefficient [32,[35], [36], [37]]. The XRD pattern displays Li1−xNixO rocksalt-type phase (3 wt%), which is a typical impurity observed together
Conclusion
The present study investigates the effect of binder and operating temperature on the electrochemical performance of V-doped LNMO electrodes tested in half-cells vs. Li. Different types of water-based binders and wt% ratios (1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, and 10 wt%) are investigated, i.e., guar gum (GG), carboxymethyl cellulose (CMC). All the results are compared with those obtained with a conventional polyvinylidene-difluoride (PVdF) binder. The electrochemical results show that even 2 wt%
CRediT authorship contribution statement
Tayfun Kocak: Conceptualization, Methodology, Investigation, Writing – original draft, Writing – review & editing. Xiadong Qi: Data curation. Xiaogang Zhang: Conceptualization, Supervision, Project administration.
Declaration of Competing Interest
None.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (U1802256,21773118, 21875107), Key Research and Development Program in Jiangsu Province (BE2018122), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors would also like to thank Dr. Burak Aktekin (Justus-Liebig-Universität Gießen), Kangsheng Huang (Nanjing University of Aeronautics and Astronautics), Dr. Francesca De Giorgio (CNR-ISMN Via Piero Gobetti) and Dr. Anti
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