Surface/Chemically Modified LiMn2 O 4 Cathodes for Lithium-Ion Batteries

A. M. Kannan; A. Manthiram

doi:10.1149/1.1482198

Lithium-ion cells currently use mostly the layered cathodes, but Co is relatively toxic and expensive. Also, only 50% of the theoretical capacity of could be practically utilized (140 mAh/g), and the highly oxidizing nature of the couple poses safety concerns at deep charge. These difficulties have created enormous worldwide interest to develop alternative cathode hosts. In this regard, the spinel has become appealing because manganese is inexpensive and environmentally benign.¹ ² Unfortunately, exhibits capacity fade during cycling and the fade is severe especially at elevated temperatures Several factors such as manganese dissolution,³ ⁴ ⁵ ⁶ formation of oxygen deficiency,⁷ electrolyte decomposition,⁸ Jahn-Teller distortion,⁹ cation mixing between lithium and manganese,¹⁰ and loss of crystallinity during cycling¹¹ have been reported to be responsible for the capacity fade.

Experiments such as cationic substitutions for manganese have been found to improve the capacity retention at room temperature.¹² ¹³ ¹⁴ ¹⁵ ¹⁶ ¹⁷ ¹⁸ However, the capacity fade at elevated temperatures could not be overcome fully. Recently, a coating of surface with ¹⁹ ²⁰ and ²¹ has been reported to improve the high temperature performance. For example, a capacity fading of about 0.08% per cycle over 100 cycles has been found at 55°C and C/5 rate for the -coated sample. With an aim to improve further the capacity retention of the spinel oxide, we have investigated the surface/chemical modification of with a variety of compositions such as and MgO; the electrochemical data including aging results are presented here. Particularly, the modification with is found to show excellent capacity retention with a capacity fade of <0.03% per cycle over 100 cycles at 60°C and C/2 rate.

Experimental

The surface/chemical modification was accomplished by treating a commercially available powder (Carus Chemical Company) with various amounts of the precursor solutions of or MgO through a chemical process so that the amount of the modifying material in the final product is 3-5 wt %.

The modification with and involved a dissolution of the carbonates or acetates of the precursor metal ions in glacial acetic acid, refluxion of the mixture for about an hour, dispersion of the spinel oxide in the precursor solution, evaporation of the solvent, and decomposition of the resultant product at around 400°C, followed by firing at 850°C in a flowing oxygen atmosphere for 12 h.

The modifications with and MgO involved the dispersion of the spinel oxide in an aqueous solution of aluminum or magnesium nitrate, precipitation of hydrous aluminum or magnesium oxide over particles through the addition of ammonium hydroxide, and heating the resultant product at 300 and 600°C, respectively, in air for 4 h.

Cathodes were fabricated with the surface/chemically modified and unmodified powder, Denka black carbon, and polytetrafluoroethylene (PTFE) binder in a weight ratio of 75:20:5. The electrochemical performances were evaluated with CR2032 coin cells assembled with the cathodes thus fabricated, metallic lithium anodes, polyethylene separators, and 1 M in ethylene carbonate (EC) and diethyl carbonate (DEC) electrolyte between 3.5 and 4.3 V. Aging experiments were carried out as follows. After the first two cycles at a current density of 0.5 mA/cm² (C/2 rate), the coin cells were discharged to DOD (depth of discharge) values of 60 and 70% in the third cycle at room temperature. The cells were then subjected to aging at a constant temperature of 55°C for 100 h and the remaining capacity was determined by discharging the aged cells at room temperature to a cutoff voltage of 3.5 V at C/2 rate. Cycled cathodes after 100 cycles were removed from the coin cells in discharged state, washed with propylene carbonate (PC), and characterized by X-ray diffraction (XRD).

Results and Discussion

The unmodified and surface/chemically modified spinel cathodes showed similar charge/discharge profiles. Figure 1 compares the cyclability data at room temperature of the surface/chemically modified cathodes with that of unmodified at a current density of 0.5 mA/cm² (C/2 rate). The surface/chemically modified samples exhibit much better capacity retention than the unmodified sample. The percent capacity fading (over 100 cycles) calculated from the discharge capacity values are given in Table I. Among all the samples, the -modified exhibits superior performance with a capacity fading rate of less than 0.02% per cycle over 100 cycles. However, the -modified sample exhibits lower initial capacity (111 mAh/g) than the unmodified (127 mAh/g). On the other hand, the -modified sample exhibits higher initial capacity (131 mAh/g) than the unmodified with a capacity fading of less than 0.05% per cycle over 100 charge/discharge cycles. Also, the -modified sample shows slightly higher capacity (123 mAh/g) than that of the -modified sample (118 mAh/g).

**Figure 1.** Cyclability data of (a) (b) -modified (c) -modified (d) -modified (e) -modified and (f) MgO-modified at a current density of 0.5 mA/cm² (C/2 rate) at room temperature.

Table I.

Specific capacity and percent of capacity fading data of surface/chemically modified and unmodified cathodes.
Sample	Room temperature			60°C
Sample	Firstdischargecapacity(mAh/g)	100thdischargecapacity(mAh/g)	Percent ofcapacityfading percycle	Firstdischargecapacity(mAh/g)	100thdischargecapacity(mAh/g)	Percent ofcapacityfading percycle
Unmodified (C/2)	126.5	97.1	0.232	132.8	78.4	0.409
Unmodified (2C)	118.0	86.7	0.266	—	—	—
-modified (C/2)	123.4	115.6	0.063	124.4	114.4	0.080
-modified (C/2)	117.5	114.3	0.027	113.1	104.6	0.075
-modified (2C)	105.7	101.4	0.040	—	—	—
-modified (C/2)	110.8	108.7	0.019	111.4	108.3	0.028
-modified (2C)	103.7	100.1	0.035	—	—	—
-modified (C/2)	131.2	124.7	0.049	130.3	109.2	0.162
MgO-modified (C/2)	136.5	126.6	0.073	—	—	—

Figure 2 compares the cyclability data at 60°C of the modified and unmodified samples at C/2 rate and the capacity fading rates are given in Table I. The surface/chemically modified cathodes exhibit much lower fading rates at elevated temperatures also compared to a fading rate of 0.41% per cycle for the unmodified Among all the modified samples, the -modified cathode exhibits the lowest fading rate of less than 0.03% per cycle over 100 cycles at 60°C. This capacity retention is superior compared to the 0.08% fade rate observed¹⁹ ²⁰ before and in this study for the -modified sample. Among the various samples listed in Table I, the -modified provides the best combination of high capacity and good cyclability.

**Figure 2.** Cyclability data of (a) (b) -modified (c) -modified (d) -modified and (e) -modified at a current density of 0.5 mA/cm² (C/2 rate) at 60°C.

Table I also compares the capacity fading rate at a higher current density of 2 mA/cm² (2C rate) at room temperature. The - and -modified cathodes exhibit excellent cyclability and rate capability. The percent of capacity fading values at 2C rate are 0.03 and 0.04% per cycle over 100 cycles, respectively, for - and -modified cathodes compared to 0.26% per cycle for the unmodified cathode.

Table II gives the aging test results for the modified and unmodified cathodes. The surface/chemically modified cathodes exhibit better capacity retention (lower capacity loss) than does the unmodified cathode under identical storage conditions. The capacity loss after storage is generally higher for the samples with 70% DOD compared to that for samples with 60% DOD.

Table II.

Aging test data of surface/chemically modified and unmodified cathodes.
Sample	Before storage		After storage at 55°C for 100 h
Sample	Second dischargecapacity(mAh/g)	Percent DOD in thirddischarge	Expectedcapacity(mAh/g)^a	Observedcapacity(mAh/g)	Percent of capacity loss
Unmodified	133.8	70	40.1	10.3	74.3
Unmodified	134.0	60	53.6	42.9	20.1
-modified	122.2	70	36.7	23.2	36.8
-modified	123.1	60	49.2	44.8	9.1
-modified	115.1	70	34.5	20.1	41.9
-modified	116.8	60	46.7	37.1	20.6
-modified	108.4	70	32.5	21.1	35.3
-modified	107.9	60	43.8	36.2	16.1
-modified	130.3	70	39.1	34.1	12.9
-modified	132.5	60	53.0	51.0	3.8
^a Obtained by subtracting the discharge capacity corresponding to 60 or 70% DOD in the third discharge from the discharge capacity in the second cycle.

In order to understand the origin of the better capacity retention for the surface/chemically modified samples, we have characterized the cycled cathodes by XRD. The XRD patterns of the unmodified and modified cathodes in discharged state after cycling at 60°C over 100 cycles are compared in Fig. 3. While the unmodified spinel cathode shows significant peak broadening indicating structural degradation and a decrease in crystallinity, the surface/chemically modified samples exhibit little or no peak broadening during cycling at elevated temperatures. Similar results were also found after soaking the samples in the electrolyte (1 M in EC and DEC) at 55°C. The peak broadening has recently been attributed by our group to the development of microstrain in the lattice, which is suppressed by cationic substitutions.²² It has been reported before by Amatucci et al.¹⁵ that the crystallinity decreases proportionately with the percent of capacity loss. The surface/chemical modification of appears to suppress the attack by the acidic species present in the electrolyte as well as the occurrence of Jahn-Teller distortion on the surface⁹ and thereby leads to better structural integrity with low microstrain and good capacity retention during cycling.

**Figure 3.** XRD patterns of (a) (b) -modified, (c) -modified, (d) -modified, and (e) -modified cathodes after 100 cycles at 60°C at a current density of 0.5 mA/cm² (C/2 rate).

**Figure 3.** XRD patterns of (a) (b) -modified, (c) -modified, (d) -modified, and (e) -modified cathodes after 100 cycles at 60°C at a current density of 0.5 mA/cm² (C/2 rate).

Finally, while the samples heated at around 300°C (-modified ) can be considered as mainly surface modified, those heated at higher temperatures of around 850°C (-modified ) can be considered as chemically modified since the guest ions will diffuse into the bulk of the host at the higher processing temperatures to give phases such as ( and Ni). This conclusion is supported by preliminary high resolution transmission electron microscopic (HRTEM) data and further TEM experiments are currently in progress. Additionally, the -modified sample that is heated at around 300°C is likely to consist of AlOOH rather than Since the amount of the modifying material is small (<5 wt %), the exact form could not be determined by XRD; further TEM work will help to clarify this.

Conclusions

spinel oxide has been surface/chemically modified with various compositions such as and MgO. The surface/chemically modified samples show better capacity retention at both room and elevated temperatures and after extended aging at 55°C in partially discharged state than did the unmodified sample. Particularly, the -modified sample shows superior capacity retention with a fade rate of <0.03% per cycle over 100 cycles at 60°C. The better performance of the surface/chemically modified cathodes is due to the maintenance of low microstrain and better structural integrity and crystallinity during cycling. We believe the modified spinel oxides may offer better long-term cyclability characteristics and safety features than the commercially used cathodes. The lower cost coupled with high rate capability and excellent cycling properties may make the surface-modified cathodes attractive particularly for electric vehicles.