Surface modification of the Co-free LiNi0.5 Mn1.5O4-δ positive electrode material for high-voltage lithium-ion batteries

Abstract

The aim of this thesis was to modify the surface of the LNMO powder to improve its electrochemical performance as a high-voltage lithium-ion battery positive electrode material. Different types of LNMO powders, synthesis routes and surface modification materials were explored to achieve this target. The Ti-containing surface modification materials were shown to be effective in improving the LNMO electrochemical performance:  Ti surface doped LNMO nanopowders synthesized via the hydrolysiscondensation approach improved the LNMO cyclic stability, Coulombic efficiency and rate performance compared to bare LNMO  Amorphous-LTO surface modified LNMO micron-powders synthesized via the solution-gel approach improved the LNMO rate performance, reduced the cell impedance during cycling and improved the cyclic stability Returning to the questions introduced in the scope of the thesis in Section 1.12:  A too high LNMO surface area results in low capacities due to a high amount of side reactions for the LNMO commercial nanopowders, as in Chapter 2. A micron-sized LNMO composed of large, irregular aggregates is not ideal either, as in Chapter 3. Optimizing the pre-calcination conditions during the aqueous solution-gel synthesis route in Chapter 3 enabled synthesis of an optimum LNMO particle size and morphology with good electrochemical performance.  To reach an optimum electrode preparation protocol and electrochemical performance with a specific powder, the solvent concentration and slurry mixture viscosity, therefore the electrode morphology, should be optimized. Several electrode preparation parameters were studied in Chapter 4 and an optimum preparation protocol is obtained.  Materials of strong metal-oxygen bonds are interesting candidates for surface modification, to improve the LNMO surface stability at high voltages. Surface doping occurs instead of surface coating, especially if high synthesis temperatures are used. The surface doping approach is an effective way to improve the electrochemical performance. Interesting candidates also include amorphous oxides. The amorphous oxides seem to increase the side reactions at the beginning of cycling. However, as the cycling continues, they might be providing a more stable, compact CEI layer, since the cell resistance drops and cycle life improves.  Two different synthesis approaches provide successful surface modifications for the nano/micron-sized LNMO powders and improve the electrochemical performance: The hydrolysis-condensation approach (Section 1.9.3.2) coupled with LNMO nanopowders in Chapter 2. It provides uniformly doped LNMO nanopowder surfaces. An electrostatic adsorption mechanism is proposed to take place during synthesis. Electrically charged LNMO surfaces apply electrostatic forces to ions/nanoparticles in the solution over large distances and loosely bind them to their surface. o The solution-gel approach (Section 1.9.4) coupled with LNMO micronpowders in Chapter 5. The solution-gel approach allows synthesis of multi-metal ion surface modification materials with controlled stoichiometry. It provides coatings/islands or surface dopants on LNMO micron-powders. A surface complex formation mechanism is proposed to take place during synthesis.  Possible reasons for the electrochemical performance improvements with the Ti-based surface modification materials are: o Surface structure stabilization by incorporation of the strong Ti-O bonds o Increased LNMO surface area after the surface modification leading to lower polarization and improved rate performance o A more favorable CEI layer formation on the electrode during cycling The findings of this thesis are summarized below in further detail: The thesis started with the use of commercial, nano-sized LiNi0.5Mn1.5O4-δ powder in Chapter 2. A hydrolysis-condensation approach was used for surface modification, followed by annealing. The surface of the LNMO powder was modified by Ti cation doping over 2-4 nm depth, while maintaining the initial spinel structure, using a hydrolysis-condensation approach followed by 500oC anneal. Particle size and surface area of the bare and surface modified LNMO remained similar after 500oC anneal and the Ti doped surface remained intact. Although the initial discharge capacity was slightly reduced, cycle life, Coulombic efficiency and rate performance were improved for Ti surface doped LNMO annealed at 500oC compared to bare LNMO also annealed at 500oC. The improvement is probably due to surface structure stabilization by the stronger Ti-O bonds, which reduces the manganese dissolution. On the other hand, during an 800oC anneal, Ti diffused from the surface towards the core of LNMO, causing a secondary LiNi0.5-xMn1.5- yTizO4 phase formation and particle size growth. Mn-Ni ordering in the lattice increased with 800oC annealing in oxygen for both bare and surface modified LNMO samples, compared to 500oC annealed samples in oxygen. However, no significant improvement was observed in cycle life or Coulombic efficiency of Ti surface modified LNMO annealed at 800oC compared to bare LNMO also annealed at 800oC. This is probably because the Ti doped surface layer of LNMO was in this case not well preserved during Ti diffusion and particle size growth. The Ti surfacedoped LNMO annealed at 500oC, having a well preserved spinel surface structure and a disordered Mn-Ni distribution, could be an interesting candidate as a cathode material for lithium-ion battery applications requiring both good cycle life and rate performance. The thesis continued with the synthesis of LNMO powders to achieve an optimum particle size, morphology and electrochemical performance in Chapter 3. The LNMO particle size and morphology were controlled using aqueous solution-gel synthesis with different pre-calcination temperatures, times and oven types. Crystalline LNMO powder morphology and particle sizes were shown to depend on the organic residue in the LNMO precursor powder before the 900oC calcination step. Calcining the LNMO precursor gel at 200oC for 40 h in a forced convection oven (LNMO-4) started a vigorous decomposition reaction and resulted in a voluminous, foam-like precursor powder morphology. The amount of organic residues before crystallization was minimized in the LNMO-4 precursor powder, enabling a small particle size after the 900oC calcination step with well-defined facets. LNMO-4 provided the highest initial discharge capacity of 121 mAh/g at 0.2 C compared to other LNMO powders. On the other hand, organic removal was probably incomplete with 170oC, 24 h, natural-convection oven pre-calcination during LNMO-1 precursor powder synthesis, resulting in large aggregates with non-uniform size distribution and poor electrochemical performance. The carboxylates or carbonaceous residues present in LNMO-1 precursor powder possibly adsorb on the surface of small metal oxide nuclei cause agglomeration and prevent formation of well-defined facets during the 900oC calcination step. Ball-milling of crystalline LNMO powder (LNMO-3) reduced the agglomeration and particle size, increased the disordering, Mn3+ concentration and lattice parameter. However, the initial discharge capacities of LNMO-3 were lower compared to LNMO-4, which was linked to the increased surface area, Mn3+ concentration and side-reactions. LNMO particle size optimization via controlling the pre-calcination conditions is more advantageous compared to size reduction via ball-milling, in terms of preserving well-defined facets, a high capacity and high Coulombic efficiency. The synthesized LNMO powders in Chapter 3 were used to optimize the electrode properties in Chapter 4, which also has an important influence on the electrochemical properties. An optimized electrode preparation protocol was proposed for the synthesized LNMO active material of ~30 μm average aggregate and ~3.5 μm average primary particle size, after individually examining the effects of composite electrode processing parameters on the electrochemical performance of the Li|LNMO cells. A good rate performance was obtained for LNMO electrodes made using a 150 μm wet thickness, 3 wt. % PVDF-NMP mixture and C65 carbon black. The use of C45 carbon black can improve the initial discharge capacity, reduce the amount of parasitic side reactions and improve the cyclability compared to C65. However, the application of C65 should be preferred, if a higher rate performance is desired. Thinky planetary mixing method coupled with a LNMO-carbon black dry mixing step could be preferred over ball-milling to homogeneously distribute the carbon black particles and increase porosity while eliminating LNMO particle size, crystalline structure or morphology changes. A calendering step should be applied to optimize the porosity, improve the electrical contact and reduce the cell impedance. However, LNMO particle size and shape should be considered while calendering and excessive forces should be avoided since LNMO particles could break apart during calendering resulting in a lower capacity. The optimized LNMO synthesis and electrode making routes in Chapters 3 and 4 were used to explore the influence of amorphous LTO surface modification of LNMO on the electrochemical performance in Chapter 5. LNMO surface was modified with amorphous LTO material (LNMO@LTO-200oC) via a solution-gel route, resulting in Ti-rich amorphous coatings/islands or Ti-rich spinel surfaces mostly on the {001} surfaces of LNMO. Transition metal ions on LNMO powder surfaces partly dissolved into the aqueous, citric acid LTO precursor solution during the LNMO@LTO-200oC synthesis. The dissolved TM ions precipitated together with the Ti ions in the LTO precursor solution during the water removal step and formed the Ti-Ni-Mn-O containing amorphous nanoparticles inside the LNMO@LTO-200oC powder. Upon 500oC annealing in dry air flow (LNMO@LTO500oC), the amorphous matter crystallized into spinel or rock salt nanoparticles depending on the composition. Amorphous LTO surface modification slightly increased the Mn3+ concentration in LNMO based on the capacity curve measurements and dQ/dV plots, but the electronic or bulk ionic conductivities were not improved. Amorphous LTO surface modification increased the LNMO surface area by ~4 times. The rate performance and cyclability were improved for LNMO@LTO-200oC compared to bare LNMO, while crystallized LNMO@LTO-500oC showed similar rate performance and cyclabilites compared to its bare counterpart. The cell impedance increased more rapidly for the bare LNMO compared to LNMO@LTO-200oC, while the dry air annealed samples had similar impedances after 1000 cycles. Amorphous coating-HF scavenging reactions might be occuring slowly on the LNMO@LTO-200oC powder surfaces during cycling, providing a more favorable CEI layer formation on the electrode surface compared to bare LNMO. ZrO2-SiO2 surface modification materials were explored for LNMO in Chapter 6 via the hydrolysis-condensation approach. ZrO2 surface modified LNMO was synthesized using different ZrO2 loadings. A too thick, tetragonal ZrO2 coating layer was probably synthesized on LNMO using 0.2 mL NH3 (25 wt. %) and 4 h annealing time, which impedes the Li+ transport and causes a large capacity drop. ZrO2 coating made using 0.1 mL NH3 (25 wt. %) and 4 h annealing time on the other hand provided probably a more optimum coating thickness, a slightly improved initial capacity but a deteriorated cyclic stability. The cyclic stability was improved using a longer anneal time of 10 h, which probably promotes Zr ion doping into the LNMO surface structure and improves the CEI layer stability during cycling at room temperature. Cycling temperature was increased to 55oC to increase the amount of side-reactions and to observe the influence of the ZrO2 surface modification layer better. Improved Coulombic efficiency values were recorded for the surface modified LNMO compared to the bare LNMO. This could be explained by a HF-scavengering mechanism where Zr cations react with HF to form a more stable, ZrF4 containing CEI layer. SiO2 was incorporated on the LNMO surface together with the ZrO2. Lower or higher temperatures were used to synthesize ZrO2-SiO2 coated or Zr-Si doped LNMO surfaces. Neither showed an important improvement in the rate performance. However, when also coupled with different cooling rates, important variations in the Ni/Mn disordering were observed for the bare/ZrO2-SiO2 coated LNMO powders, which greatly influenced the electrochemical performances. The 700oC annealed, slow cooled (1oC/min) bare/ZrO2-SiO2 surface modified LNMO powders showed a drastic increase in the Ni/Mn ordering and better electrochemical performance compared to the 500oC annealed, furnace cooled bare/surface modified LNMO powders. Increased ordering is probably caused by the slower cooling rate in oxygen flow. The higher temperature used on the other hand could be introducing more oxygen vacancies in the structures, because of the temperature dependent O2 evolution reaction. As a result, a more optimum amount of oxygen vacancies and Ni/Mn ordering were probably achieved in the bare/ZrO2-SiO2 surface modified LNMO powders with the 700oC annealing and 1°C/min heating/cooling rates, which helped improve the electrochemical performance.