The development of environmentally friendly, low cost and safe lithium-ion batteries (LIBs) with increased energy and power densities is one of the key challenges to boost the progress of sustainable transport. The energy density of a battery can be enhanced by using high-voltage and/or high-capacity cathode materials.

LiNi_0.5Mn_1.5O₄ (LNMO) is one of the most promising high-voltage cathode materials for its high theoretical specific capacity of 146.7 mAh g^-1 and high nominal operating voltage of 4.7 V vs. Li⁺/Li [1]. Combining LNMO with a graphite anode should allow full cells with specific energies higher than 200 Wh kg^-1, in order to extend the electric driving range of hybrid electric vehicles (HEVs). However, despite the LNMO's appealing properties, e.g., low cost, environmental friendliness, excellent rate capability and good safety, the major concern that limits the use of this material is its reactivity towards conventional electrolytes, which are prone to decompose at high potentials leading to the formation of a thick surface layer and capacity loss, both being detrimental for the cycling performance of the electrode. Since advanced electrolytes stable over 5 V are under investigation but not yet available [2], several strategies have been pursued to address the interface instability issues, such as the partial substitution of Ni and Mn by dopants and surface modifications [3], the coating with reduced graphene oxides to enhance conductivity and protect the electrode from the reaction with the electrolyte [4-6], and the use of additives in conventional electrolytes [2].

The water-soluble binder, namely sodium carboxymethylcellulose (CMC), is here proposed instead of polyvinylidene fluoride (PVdF), the most common binder in LIBs, for the making of LNMO-composite electrodes. CMC is more eco-friendly than PVdF and appears to be a very promising alternative binder for high-voltage cathode materials [7], even for LiNi_0.4Mn_1.6O₄ [8] and LiNi_0.5Mn_1.5O₄ [9].

This study demonstrates the strong impact of the CMC binder on LNMO-composite electrodes that perform notably better than LNMO-PVdF electrodes, especially upon long cycling, showing higher capacity retention in half cells vs. Li/Li⁺ in conventional electrolyte (EC:DMC – LiPF₆), both at 20° and 40°C. CMC promotes a homogeneous carbon dispersion that may improve the percolating network and, hence, the cycling performance of the electrodes. Therefore, CMC prevents the unwanted reactions between the electrode and electrolyte upon high-operating voltage, as also evinced by the ex-situSEM and XPS characterization of cycled electrodes.

Furthermore, graphite//LNMO-CMC full cell was assembled and tested according to the DOE battery test for plug-in HEVs and the preliminary and very promising results are here reported.

References

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