Xuemei Zhao

A High Precision Coulometry Study of the SEI Growth in Li/Graphite Cells

The charge and discharge endpoint capacities as well as the coulombic efficiency of Li/graphite coin cells have been examined using the high precision charger at Dalhousie University. Cells were charged and discharged at different C-rates and temperatures to observe trends in the formation of the solid electrolyte interphase (SEI) on the graphite electrode. The experiments show that time and temperature, not cycle count, are the dominant contributors to the growth of the SEI. The charge consumed by the SEI and hence the SEI thickness, increase approximately with time 1/2 consistent with a process where the temperature-dependent SEI growth rate is inversely proportional to the SEI thickness. The charge consumed by the SEI is proportional to the electrode surface area and this increased consumption on high surface area electrodes continues during cycling, at least with the 1 M LiPF 6 ethylene carbonate:diethyl carbonate electrolyte used.

Synthesis, Electrochemical Properties, and Thermal Stability of Al-Doped LiNi1 ∕ 3Mn1 ∕ 3Co ( 1 ∕ 3 − z ) Al z O2 Positive Electrode Materials

Al-doped Li[Ni 1/3 Mn 1/3 CO (1/3-z) Al z ]O 2 (0 ≤ z ≤ 0.14) samples were synthesized using a coprecipitation method followed by calcination with LiOH·H 2 O at 500°C for 3 h and 900°C for 3 h. Electrochemical testing showed that Li[Ni 1/3 Mn 1/3 Co (1/3-z) Al z ]O 2 had a high initial discharge capacity and good charge-discharge cycle performance in the voltage ranges of either 2.5-4.3 or 2.5-4.6 V. The impact of Al doping on the thermal stability of Li[Ni 1/3 Mn 1/3 Co (1/3-z) Al z ]O 2 was studied by accelerating rate calorimetry. Al substitution for Co in Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 caused a dramatic improvement in thermal stability over the material without aluminum. Al-doped Li{Ni 1/3 Mn 1/3 Co (1/3-z )Al z ]O 2 with z = 0.1 displays an energy density greater than LiMn 2 O 4 and equivalent or higher thermal stability, making it a possible choice as an electrode material for large Li-ion batteries.

Relative Impact of Al or Mg Substitution on the Thermal Stability of LiCo1 − z M z O2 (M = Al or Mg) by Accelerating Rate Calorimetry

Al- or Mg-substituted LiCo 1―z M z O z (M = Al or Mg) (z = 0, 0.05, 0.1) samples were synthesized by a coprecipitation-based method followed by heating with LiOH·H 2 O at 500°C for 3 h and again at 900°C for 3 h. Electrochemical studies showed that Al or Mg substitution reduces the initial discharge capacity of LiCo 1―z M z O z cells but does not strongly impact their cycle life. The effects of Al or Mg substitution on the thermal stability of charged LiCo 1―z M z O 2 were studied by accelerating rate calorimetry and indicate that Al substitution is far more effective than Mg substitution in improving the thermal stability of LiCo 1―z M z O 2 .

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 .

Impact of Al or Mg substitution on the Thermal Stability of Li1.05Mn1.95 − z M z O4 (M = Al or Mg)

Single phase Al or Mg-substituted Li 1.05 Mn 1.95-z M z O 4 (M = Al or Mg) (z = 0, 0.05, 0.1, 0.15, 0.2) samples were synthesized by a two-step solid-state reaction. Electrochemical studies confirmed that Al or Mg substitution reduced the reversible specific capacity of Li 1.05 Mn 1.95-z M z O 4 . The effects of Al or Mg substitution on the thermal stability of charged Li 1.05 Mn 1.95-z M z O 4 samples in the presence of a 1 M LiPF 6 ethylene carbonate:diethyl carbonate electrolyte were studied by accelerating rate calorimetry. The results showed that Al or Mg substitution has little effect on improving the thermal stability of Li 1.05 Mn 1.95-z M z O 4 . This is in contrast to the layered lithium transition-metal oxides (e.g., Li[Co 1-z Al z ]O 2 , Li[Ni 1-z Al z ]O 2 , Li[Ni 0.8 Co 0.2-z Al z ]O 2 , and Li[Ni 1/3 Mn 1/3 Co 1/3-z Al z ]O 2 ], where Al substitution significantly lowers the kinetics of the reactions of the charged electrode materials with electrolyte.

Phases Formed in Al-Doped Ni1 / 3Mn1 / 3Co1 / 3 ( OH ) 2 Prepared by Coprecipitation: Formation of Layered Double Hydroxide

The coprecipitation process, using either mixed nitrate or sulfate solutions, to produce Al-doped Ni 1/3 Mn 1/3 Co 1/3 (OH) 2 has been studied in detail. Using X-ray diffraction (XRD) and thermal gravimetric analysis (TGA), it has been determined that charge-compensating NO - 3 or SO 2- 4 ions accompany the incorporated Al 3+ ions into the structure to form layered double hydroxides (LDHs). The phases formed were determined for 0 1/6 are single phase Ni 1/3 Mn 1/3 Co 1/3-z Al z (OH)2(NO 3 ) 1/6 (OH) z-1/6 . TGA experiments show that the NO - 3 in the NO - 3 -containing LDH is eliminated at lower temperature than the SO 2- 4 in the SO 2- 4 -containing LDH. Thus coprecipitations using nitrates may be preferred in situations where excellent cation mixing is desired in the production of Al-doped Li[Ni 1/3 Mn 1/3 Co 1/3-z Al]O 2 .

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.

Comparison of Thermal Stability Between Lithiated Sn30Co30C40, LiSi, or Li0.81C6 and 1 M LiPF6 EC:DEC Electrolyte at High Temperature

The reactions between lithiated Sn 30 Co 30 C 40 , LiSi, or Li 0.81 C 6 and 1 M LiPF 6 ethylene carbonate: diethyl carbonate (EC:DEC) electrolyte were compared using accelerating rate calorimetry (ARC). Both lithiated Sn 30 Co 30 C 40 and LiSi showed less reactivity than Li 0.81 C 6 over the entire ARC test temperature range. For both lithiated Sn 30 Co 30 C 40 and LiSi, the self-heating rates were very similar in the low-temperature range (below 225°C). However, lithiated Sn 30 Co 30 C 40 was less reactive than LiSi at higher temperatures primarily because the formed Si reacted with the electrolyte itself, while the formed Sn-Co-C nanocomposite apparently does not. The attractive thermal stability of lithiated Sn 30 Co 30 C 40 suggests that it could be a good alternative to currently used carbonaceous negative electrode materials for lithium-ion batteries where increased safety is desired.

Studies of LiNi2 / 3Mn1 / 3O2: A Positive Electrode Material That Cycles Well to 4.6 V

LiNi 2/3 Mn 1/3 O 2 was synthesized from LiOH·H 2 O and Ni 2/3 Mn 1/3 (OH) 2 mixed hydroxide. Four different coprecipitation routes were used to prepare the Ni 2/3 Mn 1/3 (OH) 2 precursor beginning with divalent metal nitrates or metal sulfates. When ammonia was added during the coprecipitation under anaerobic conditions, spherical and dense particles of Ni 2/3 Mn 1/3 (OH) 2 were prepared. The impact of various coprecipitation methods on the subsequent properties of LiNi 2/3 Mn 1/3 O 2 was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical tests. Spherical and dense particles of LiNi 2/3 Mn 1/3 O 2 showed an excellent charge-discharge cycle life at 4.6 V. The impact of heating temperature and time on the properties of LiNi 2/3 Mn 1/3 O 2 was also studied by XRD, SEM, and electrochemical tests and suggested that the heating temperature plays a significant role in determining the electrochemical performance of LiNi 2/3 Mn 1/3 O 2 . Accelerating rate calorimetry results suggest that LiNi 2/3 Mn 1/3 O 2 is less thermally stable than Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 .

Advantages of Simultaneous Substitution of Co in Li [ Ni1 ∕ 3Mn1 ∕ 3Co1 ∕ 3 ] O2 by Ni and Al

Ni and Al can be substituted for Co in Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 by increasing y and z in x LiNi 1/3 Mn 1/3 Co 1/3 O 2 -y LiNi 2/3 Mn 1/3 O 2-z LiNi 1/3 Mn 1/3 Al 1/3 O 2 (x + y + z = 1) pseudoternary solid solution systems. Additions of Al improve the thermal stability of the charged positive electrode material, at a cost of capacity reduction, which can be at least partially recovered by the addition of Ni. The composition Li[Ni (1/3+0.07) Mn 1/3 CO (1/3-0.2) Al 0.13 ]O 2 (x = 0.4, y = 0.2, z = 0.4) shows excellent thermal stability compared to Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 , but similar specific capacity. The samples were synthesized via a coprecipitation-based synthetic method to prepare a mixed hydroxide precursor, followed by sintering with a Li salt. Such materials may find application in large-size Li-ion batteries targeted for vehicle applications.