Nanostructured materials for lithium-ion batteries: Surface conductivity vs. bulk ion/electron transport

Abstract

Lithium metal phosphates are amongst the most promising cathode materials for high capacity lithium-ion batteries. Owing to their inherently low electronic conductivity, it is essential to optimize their properties to minimize defect concentration and crystallite size (down to the submicron level), control morphology, and to decorate the crystallite surfaces with conductive nanostructures that act as conduits to deliver electrons to the bulk lattice. Here, we discuss factors relating to doping and defects in olivine phosphates LiMPO4 (M = Fe, Mn, Co, Ni) and describe methods by which in situ nanophase composites with conductivities ranging from 10(-4)-10(-2) S cm(-1) can be prepared. These utilize surface reactivity to produce intergranular nitrides, phosphides, and/or phosphocarbides at temperatures as low as 600 degrees C that maximize the accessibility of the bulk for Li de/insertion. Surface modification can only address the transport problem in part, however. A key issue in these materials is also to unravel the factors governing ion and electron transport within the lattice. Lithium de/insertion in the phosphates is accompanied by two-phase transitions owing to poor solubility of the single phase compositions, where low mobility of the phase boundary limits the rate characteristics. Here we discuss concerted mobility of the charge carriers. Using Mössbauer spectroscopy to pinpoint the temperature at which the solid solution forms, we directly probe small polaron hopping in the solid solution Li(x)FePO4 phases formed at elevated temperature, and give evidence for a strong correlation between electron and lithium delocalization events that suggests they are coupled.

Electrochemical Performance and Delithiation/Lithiation Characteristics of Mixed LiFe1-yMyPO4(M= Co, Ni) Electrode Materials

“…This is due to carbothermal reduction of the phosphate phase when heated in argon atmosphere with carbon containing precursors or carbon additive, resulting in production of secondary phosphide phases. 58,59 These phosphide phases are highly electrically conductive, 20,39,59 and thus increase the conductivity of the sample. Small phosphide contents are found to even improve the discharge capacity, 20,22 but higher amounts (>5%) lead to performance deterioration, 20 as the phosphides are electrochemically inactive and may prevent Li + -ion transfer at the particle surface.…”

LiMg[sub x]Mn[sub 1−x]PO[sub 4]/C Cathodes for Lithium Batteries Prepared by a Combination of Spray Pyrolysis with Wet Ballmilling

“…This disadvantage has been successfully overcome for the LiFePO 4 cathodes via the synthesis of nanosized powders, the conductive layer coating of the compound particles surface and doping with cations. [3][4][5][6][7][8][9] It was shown that in the presence of carbon in the cation-doped olivine LiFePO 4 , the carbothermal reduction of iron phosphate at high temperatures leads to the formation of phosphides at the grain boundaries, and this nanonetwork could be responsible for the grain-boundary transport enhancement.…”

“…This disadvantage has been successfully overcome for the LiFePO 4 cathodes via the synthesis of nanosized powders, the conductive layer coating of the compound particles surface and doping with cations. [3][4][5][6][7][8][9] It was shown that in the presence of carbon in the cation-doped olivine LiFePO 4 , the carbothermal reduction of iron phosphate at high temperatures leads to the formation of phosphides at the grain boundaries, and this nanonetwork could be responsible for the grain-boundary transport enhancement.9,10 The same process could be readily extendable to other olivines such as LiMnPO 4 .9 Chung et al 11 showed that by selective doping with cations supervalent to lithium, lattice electronic conductivity of LiFePO 4 can be increased remarkably, which is attributed to the cation-deficient solid solution formation, and it was suggested that the same doping mechanism could be applied to other olivine structured materials.These facts encourage great attention to another olivine structured material, LiMnPO 4 , which is more attractive than LiFePO 4 because of higher theoretical energy density due to the higher operating voltage of about 4 …”

“…Among other candidates to replace the high cost and toxic cobalt-based cathodes for LIBs, the olivine structured phosphates LiMPO 4 ͑M = Fe, Mn, Ni͒ are the most promising ones due to the reliable combination of theoretical capacity and cost, nontoxicity, and high electrochemical and thermal stabilities. [2][3][4][5] However, high ionic and electronic resistance of these materials restricts achieving high electrochemical activity. This disadvantage has been successfully overcome for the LiFePO 4 cathodes via the synthesis of nanosized powders, the conductive layer coating of the compound particles surface and doping with cations.…”

Multi-Scale Thermo-Electrochemical Modeling of Performance and Aging of a LiFePO4/Graphite Lithium-Ion Cell

“…[52][53][54] Although pure lithium iron phosphate is a very bad electron conductor, [55][56][57] thanks to carbon coating or conductive additives used in commercial LFP batteries the electrode is likely to have one order of magnitude higher conductivity than the electrolyte. 51,[58][59][60][61][62] Boundary conditions for the species conservation are J i = 0 at both electrode/current collector interfaces. For the ionic potential, the boundary conditions are d( φ)/dx = 0 at both electrode/current collector interfaces.…”