Junwei Jiang

High-Rate Overcharge Protection of LiFePO4-Based Li-Ion Cells Using the Redox Shuttle Additive 2,5-Ditertbutyl-1,4-dimethoxybenzene

LiFePO 4 /Li 4 / 3 Ti 5 / 3 O 4 Li-ion cells have been investigated by many groups and their behavior in standard electrolytes such as 1 M LiPF 6 ethylene carbonate: diethyl carbonate (EC:DEC) is well known. Here we report on the behavior of these cells with 2,5-ditertbutyl-l,4-dimethoxybenzene added to the electrolyte as a redox shuttle additive to prevent overcharge and overdischarge. We explore methods to increase the current-carrying capacity of the shuttle and explore the heating of practical cells during extended overcharge. The solubility of 2,5-ditertbutyl-l,4-dimethoxybenzene was determined as a function of salt concentration in lithium bis-oxolatoborate-(LiBOB) and LiPF 6 -containing electrolytes based on propylene carbonate (PC), EC, DEC, and dimethyl carbonate (DMC) solvents. Concentrations of 2,5-ditertbutyl-l,4-dimethoxybenzene up to 0.4 M can be obtained in 0.5 M LiBOB PC:DEC (1:2 by volume). Coin-type test cells were tested for extended overcharge protection using an electrolyte containing 0.2 M 2,5-ditertbutyl-1,4-dimethoxybenzene in 0.5 M LiBOB PC:DEC. Sustained overcharge protection at a current density of 2.3 mA/cm 2 was possible and hundreds of 100% shuttle-protected overcharge cycles were achieved at current densities of about 1 mA/cm 2 . The diffusion coefficient of the shuttle molecule in this electrolyte was determined to be 1.6 X 10 - 6 cm 2 /s from cyclic voltammetry and also from measurements of the shuttle potential vs. current density. The power produced during overcharge was measured using isothermal microcalorimetry and found to be IV as expected, where I is the charging current and V is the cell terminal voltage during shuttle-protected overcharge. Calculations of the temperature of 18650-sized Li-ion cells as a function of time during extended shuttle-protected overcharge at various C-rates are presented. These show that Li-ion cells need external cooling during extended shuttle-protected overcharge if currents exceed about C/5 rates.

Studies of Aromatic Redox Shuttle Additives for LiFePO4-Based Li-Ion Cells

Fifty eight aromatic organic molecules were screened as chemical shuttles to provide overcharge protection for LiFePO 4 /graphite and LiFePO 4 /Li 4 / 3 Ti 5 / 3 O 4 Li-ion cells. The majority of the molecules were based on methoxybenzene and on dimethoxybenzene with a variety of ligands added to explore their effect. The added ligands affect the redox potential of the molecules through their electron-withdrawing effect and affect the stability of the radical cation. Of all the molecules tested, only 2,5-di-tert-butyl-1,4-dimethoxybenzene shows an appropriate redox potential of 3.9 V vs Li/Li + and long-term stability during extended abusive overcharge totaling over 300 cycles of 100% overcharge per cycle. The reasons for the success of this molecule are explored.

Comparison of the Thermal Stability of Lithiated Graphite in LiBOB EC/DEC and in LiPF6 EC/DEC

Accelerating rate calorimetry (ARC) has been used to compare the thermal stability of lithiated mesocarbon microbeads (MCMB) (Li 0 . 8 1 C 6 ) in the presence of saturated lithium bis(oxalato)borate (LiBOB) in ethylene carbonate (EC)/diethyl carbonate (DEC) (about 0.8 M) and LiPF 6 in EC/DEC (1.0 M). The exothermic reaction between lithiated MCMB and LiBOB EC/DEC electrolyte does not proceed strongly until about 170°C but it begins at about 80°C for lithiated MCMB in LiPF 6 EC/DEC. We believe that LiBOB electrolytes offer significant advantages for cell abuse tolerance compared to LiPF 6 electrolytes, at least as far as anode/ electrolyte reactions are concerned.

High-Potential Redox Shuttle for Use in Lithium-Ion Batteries

Redox shuttle additives can be used in lithium-ion cells to protect against overcharging and for cell balancing in multicell packs. Most previously reported redox shuttles have been either unstable as shuttles, resulting in a short duration of overcharge protection, or have redox potentials that make them suitable only for cells containing lower potential positive electrode materials, such as LiFePO 4 . A new molecule, l,4-di-t-butyl-2,5-bis(2,2,2-trifluoroethoxy)benzene, is shown here to be a stable redox shuttle with a redox potential of 4.25 V and provides overcharge protection in full Li-ion cells, implying adequate reductive stability. Experiments using Li/LiCoO 2 and Li/Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 cells show that this molecule can work with high-energy-density positive electrodes as well as in full cells containing LiFePO 4 positive electrodes and Li, mesocarbon microbead, or Li 4 Ti 5 O 12 negative electrodes.

Thermal Stability of 18650 Size Li-Ion Cells Containing LiBOB Electrolyte Salt

Lithium bis(oxalato)borate (LiBOB) has been proposed recently as an electrolyte salt for Li-ion batteries, however safety testing of full Li-ion cells incorporating this salt has not been reported. Earlier accelerating rate calorimetry (ARC) work demonstrated that the thermal reactivity between LiBOB ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte and Li 0.81 C 6 was lower than that between LiPF 6 EC/DEC electrolyte and the same negative electrode material, suggesting that LiBOB may be an attractive salt choice for safer Li-ion cells. Here, we report ARC studies of the reactions between LiBOB EC/DEC electrolyte and Li 0.5 CoO 2 and compare them to the reactions between LiPF 6 EC/DEC and the same positive electrode material. Unfortunately, the reactivity of LiBOB electrolyte with the charged positive electrode initiates at a substantial rate at about 40°C lower in temperature than for LiPF 6 electrolyte. Oven exposure tests on charged (4.2 V) 18650 Li-ion cells made using the same electrolytes and electrode materials show that the heat caused by the reaction of the negative electrode with electrolyte is less for LiBOB electrolyte than for LiPF 6 electrolyte but that the opposite is true for the heat caused by the reaction of the positive electrode with electrolyte, as expected based on the ARC measurements. Using ARC tests on the individual electrode reactivity and oven exposure testing on 18650 Li-ion cells, the usefulness of electrolytes with mixed LiBOB/LiPF 6 salts is also explored.

Reactivity of Li y [ Ni x Co1 − 2x Mn x ] O 2 ( x = 0.1 , 0.2, 0.35, 0.45, and 0.5; y = 0.3 , 0.5) with Nonaqueous Solvents and Electrolytes Studied by ARC

Samples of the layered cathode material, Li[Ni x Co 1 - 2 x Mn x ]O 2 (x = 0.1, 0.2, 0.35, 0.45, and 0.5), were synthesized at 900°C, fabricated into electrodes, and charged to both 4.2 and 4.4 V vs. Li/Li + . The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with ethylene carbonate (EC)/diethyl carbonate (DEC) solvent or 1 M LiPF 6 EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were welded closed. ARC shows that there is no significant exothermic reaction between Li 0 . 5 [Ni 0 . 1 Co 0 . 8 Mn 0 . 1 ]O 2 (x = 0.1, 4.2 V) and EC/DEC solvent or 1 M LiPF 6 EC/DEC until approximately 170°C, compared to a 140°C onset temperature for LiCoO 2 charged to 4.2 V. As x increases from 0.1 to 0.2, the onset temperature for materials charged to 4.2 V does not change significantly, but the main exothermic peak shifts significantly to higher temperatures. However, the thermal stability of Li[Ni x Co 1 - 2 x Mn x ]O 2 with x = 0.1 is significantly worse for samples charged to 4.4 V. By contrast, samples with x = 0.2, 0.35, 0.45, or 0.5 show almost identical excellent thermal stability at both 4.2 and 4.4 V.

Dependence of the Heat of Reaction of Li0.81C6 ( 0.1 V ) , Li7Ti5O12 ( 1.55 V ) , and Li0.5VO2 ( 2.45 V ) Reacting with Nonaqueous Solvents or Electrolytes on the Average Potential of the Electrode Material

Negative electrode materials for Li-ion batteries can be selected from several choices that have different potentials vs Li metal. In this paper, using Li 0 . 8 1 C 6 (0.1 V), Li 7 Ti 5 O 1 2 (1.55 V), and Li 0 . 5 VO 2 (B) (2.45 V) as examples, we show how the heat of reaction between these electrode materials and nonaqueous solvents or electrolytes depends on the negative electrode potential. The three fully lithiated negative electrode materials react with ethylene carbonate/diethyl carbonate solvent, producing Li 2 CO 3 , C 2 H 4 , and the delithiated phases [C, Li 4 Ti 5 O 1 2 , or VO 2 (B)]. The heat of reaction depends strongly on the negative electrode potential and is -54 ′ 8 kJ/(mol Li) for Li 0 . 5 VO 2 (B), -110 ′ 13 kJ/(mol Li) for Li 7 Ti 5 O 1 2 , and -215 ′ 16 kJ/(mol Li) for Li 0 , 8 1 C 6 , respectively. Thermodynamic considerations show that the heat of reaction per mole of lithium vs electrode potential should vary as 96.5 kJ(mol Li) - 1 V - 1 , in good agreement with experiment. These results suggest that energy of Li-ion cells can be traded for increased safety by switching to higher potential negative electrode materials.

The Effect of Al Substitution on the Reactivity of Delithiated LiNi ( 0.5 − z ) Mn ( 0.5 − z ) A12z O2 with Nonaqueous Electrolyte

The high-temperature reactions between 1 M LiPF 6 ethylene carbonate:diethyl carbonate and Al-doped LiNi (0.5-z) Mn (0.5-z) Al 2z O 2 charged to 4.3 V are studied by accelerating rate calorimetry and compared with those of charged LiNi 1/3 Mn 1/3 Co 1/3 O 2 and spinel LiMn 2 O 4 . Simultaneous Al substitution for Ni and Mn in LiNi 0.5 Mn 0.5 O 2 improves the thermal stability. The maximum self-heating rate attained and the specific capacity decrease as the Al content increases. Materials with z > 0.03 are less reactive with electrolyte than spinel LiMn 2 O 4 at all temperatures studied. There is a range of compositions near z = 0.05 that show excellent promise as materials which are both safer and more energy dense than spinel LiMn 2 O 4 .

Solid-State Synthesis as a Method for the Substitution of Al for Co in LiNi1 ∕ 3Mn1 ∕ 3Co ( 1 ∕ 3 − z ) Al z O2

Al was substituted for Co in LiNi 1/3 Mn 1/3 Co 1/3 O 2 by two different routes: during coprecipitation of the hydroxide precursor or afterward as Al(OH) 3 introduced during solid-state sintering. The latter method avoids the formation of layered double hydroxides during the coprecipitation, but may not ensure uniform distribution of the Al atoms in the mixed cation layers. The electrochemical performance and thermal stability of Al-substituted LiNi 1/3 Mn 1/3 Co (1/3-2) Al 2 O 2 was studied by coin cell testing and accelerating rate calorimetry tests, respectively. High resolution compositional analysis showed that the Al distribution was not uniform in material prepared by solid-state synthesis at 900°C, unlike materials prepared by coprecipitation at 900 or 1000°C or by solid-state synthesis at 1000°C. Nonuniform Al distribution does not affect the electrochemical properties strongly, but has a strong negative impact on the thermal stability of the charged electrode materials. The solid-state sintering substitution of A1 for Co can be successful if the sintering temperature is high enough to ensure a uniform Al distribution.

Structure, Electrochemical Properties, and Thermal Stability Studies of Li [ Ni0.2Co0.6Mn0.2 ] O2 Effect of Synthesis Route

Two Li[Ni 0 . 2 Co 0 . 6 Mn 0 . 2 ]O 2 samples were synthesized by firing Ni 0 . 2 Co 0 . 6 Mn 0 . 2 (OH) 2 coprecipitate mixed with either LiOH or Li 2 CO 3 . Another two Li[Ni 0 . 2 Co 0 . 6 Mn 0 . 2 ]O 2 samples were made from Ni 0 . 4 1 6 Co 0 . 1 6 8 Mn 0 . 4 1 6 (OH) 2 coprecipitate mixed with Co(OH) 2 and LiOH or Li 2 CO 3 . All samples were single phase by X-ray diffraction. The structure and electrochemical properties of the synthesized Li[Ni 0 . 2 Co 0 . 6 Mn 0 . 2 ]O 2 samples were compared and the reactivity of the four charged Li x [Ni 0 . 2 Co 0 . 6 Mn 0 . 2 ]O 2 (4.2 V) samples with electrolyte was examined using accelerating rate calorimetry. All four charged Li x [Ni 0 . 2 Co 0 . 6 Mn 0 . 2 ]O 2 (4.2 V) samples show less reactivity than Li x Co0 2 (4.2 V) in ethylene carbonate/diethyl carbonate solvent or in LiPF 6 -based electrolyte. However, Li[Ni 0 . 2 Co 0 . 6 Mn 0 . 2 ]O 2 synthesized from Ni 0 . 2 Co 0 . 6 Mn 0 . 2 (OH) 2 mixed with LiOH or Li 2 CO 3 shows higher thermal stability than Li[Ni 0 . 2 Co 0 . 6 Mn 0 . 2 ]O 2 samples made from Ni 0 . 4 1 6 Co 0 . 1 6 8 Mn 0 . 4 1 6 (OH) 2 coprecipitate mixed with Co(OH) 2 and LiOH or Li 2 CO 3 , even though the particle size of the latter materials is larger. The reasons for this surprising result are explained. Because the safety of Li[Ni x Co 1 - 2 x Mn x ]O 2 materials is strongly dependent on x, near x = 0, the safest materials are those with the most homogeneously mixed cations.