Influence of Synthesis, Dopants and Cycling Conditions on the Cycling Stability of Doped LiNi_0.5Mn_1.5O₄Spinels

Abstract

The high voltage LiNi 0.5 Mn 1.5 O 4 spinel suffers from severe capacity fade when cycled against a graphitic anode as well as a relatively low theoretical capacity. Using metallic lithium as counter electrode, the stability is improved and the ability of the spinel structure to host 2 Li eq. can be used to improve the capacity. This leads to a theoretical specific energy of ∼1000 Wh kg −1 . Unfortunately, the cycling of 2 Li eq. involves a phase transition from cubic to tetragonal associated with material degradation. In this work doping is used to improve capacity retention when cycling between 2.0 and 5.0 V. Initial capacities and stabilities are directly dependent on synthesis conditions and doping elements. Therefore, Fe-and Ti-doped spinels are compared with Ru-and Ti-doped spinels and tested at different cycling conditions. The cycling stability can be improved significantly by using reannealed material and by changing the discharge cutoff criteria. Thus a capacity of 190 mAh g −1 is achieved at a rate of C/2 with a capacity retention of ∼92% after 100 cycles. Furthermore, differences in the discharge behavior between the differently treated Ru-and Ti-doped materials are discussed based on the electrochemical behavior, the particle morphology and in-situ XRD analysis. The LiNi 0.5 Mn 1.5 O 4 spinel (LNMO) is a potential candidate for a high-energy cathode material in Li ion batteries. This is due to the high operating voltage of ∼ 4.7 V vs. Li + /Li. 1,2 Regardless of the high voltage the spinel offers a lower specific energy compared with other cathode materials (e.g. Li-rich layered oxides with capacities of ca. 220 -250 mAh g −1 ). 3-5However, the ability to host an additional lithium on the 16c octahedral sites 6,7 when a lithium metal anode is used, leads to an increased specific energy. The theoretical capacity increases up to 294 mAh g −1 and a theoretical energy of around 1000 Wh kg 9,10 Furthermore the trivalent manganese is Jahn-Teller active and cycling is accompanied by a strong Jahn-Teller-distortion.7,9 The c-axis parameter is increased by ∼6% while the a-and b-axis parameters shrink by ca. 0.6% leading to a phase transition from cubic to tetragonal symmetry. 11,12 The loss of manganese and the mechanical strain due to the phase transition lead to a severe capacity fade. Therefore, stable cycling of 2 Li eq. in the LNMO\\Li metal system is not possible.In order to enhance the electrochemical stability of spinel cathode materials several studies use various elements as dopants (e.g. Cr, Fe, Ga;13 Fe, Cu, Al, Mg; 14C r, Fe, Co, Ga; 15 Ti 16 ). In many publications the positive influence of Fe-and/or Ti-doping on the electrochemical stability of LNMO has been shown. 13,14,[16][17][18][19][20] Another promising doping element is Ruthenium. It has also been investigated as dopant in previous studies for the manganese spinel 21 and the LNMO spinel. 22-25It leads to an increased rate capability due to a higher electronic conductivity as well as faster lithium diffusion. However, except from z E-mai...

Electrochemical and Electronic Charge Transport Properties of Ni-Doped LiMn2O4 Spinel Obtained from Polyol-Mediated Synthesis

“…By using this setup, we recently performed local electrical transport measurements on single Fe-, Ti-doped and Ru-, Ti-doped LNMO spinel crystals and demonstrated a change in the electrical conductivity depending on the dopant, which was consistent with IS measurements on pressed pellets of the corresponding materials [34]. …”

On the electrochemical properties of the Fe–Ti doped LNMO material LiNi0.5Mn1.37Fe0.1Ti0.03O3.95

“…Figure 4a and S6), which indicates that the 8a Wyckoff site (tetrahedral voids) is occupied by Lithium exclusively. 26,27,29,43 While the lattice parameters of LNMFTO spinel could be refined very accurately, the phase fraction of the secondary phase, mostly referred to as rocksalt phase in literature, is rather difficult to determine reliably. As highlighted in Figure 4b-d, major reflections of both phases overlap, which makes it difficult to refine profile parameters independently.…”

“…1,13,24 In contrast to most prior doping studies, in the present work manganese is replaced exclusively. The doping elements Ti IV and Fe III both lead to an increases of the lattice parameter [25][26][27] , suppress the formation of cation order 25,28 and increase ionic and electronic conductivities [28][29][30] . Finally, titanium and iron are cheap and abundant doping elements and, as we will show, Fe-Ti doping allows for excellent electrochemical properties combined with very low Mn(III) content, which is an unsolved problem with undoped LNMO 17,31 .…”

Spray-Drying of Electrode Materials for Lithium- and Sodium-Ion Batteries

“…In the case of titanate or silicate electrode materials, the preparation of solutions is more difficult because few precursors are soluble in aqueous solutions of less-than-extreme pH. Chloride and/or alkoxide precursors (such as TEOS Si(OC 2 H 5 ) 4 [350,352], titanium isopropoxide Ti(OC 3 H 7 ) 4 [128,132,212,227,235,243,248,251] or tert-butoxide Ti(OC 4 H 9 ) 4 [211,222,229,238,239,240,241,249,250,265], …) can be solubilized in alcohol but hydrolysis takes place when mixing with water, leading to the precipitation of SiO 2 or TiO 2 unless special care is taken as in the strategies summarized in Figure 4.…”