From the American Chemical Society...

An Advanced Lithium Ion Battery Based on High Performance Electrode Materials

Abstract

In this paper we report the study of a high capacity Sn−C nanostructured anode and of a high rate, high voltage Li[Ni_0.45Co_0.1Mn_1.45]O₄ spinel cathode. We have combined these anode and cathode materials in an advanced lithium ion battery that, by exploiting this new chemistry, offers excellent performances in terms of cycling life, i.e., ca. 100 high rate cycles, of rate capability, operating at 5C and still keeping more than 85% of the initial capacity, and of energy density, expected to be of the order of 170 Wh kg⁻¹. These unique features make the battery a very promising energy storage for powering low or zero emission HEV or EV vehicles.

3 Results and Discussion

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3.1 The Anode

Lithium metal, Li−M, alloys (M = Sn, Si, Sb, etc.) are very appealing, advanced anode materials, due to their specific capacity that is much higher than that of commercially used graphite.(2-4) However, the use of Li−M alloys has been so far prevented by the large volume expansion−contraction experienced during their electrochemical process in lithium cells. We have shown that the volume stress issue can be efficiently solved by developing suitable electrode morphologies, such as M−C nanocomposites.(12) Indeed, we demonstrated that a Sn−C composite may operate in lithium cells with several hundred cycles, without capacity decay and with discharge (lithium-alloying)−charge (lithium dealloying) efficiency approaching 100%. As reported in a previous paper,(10) the Sn/C weight ratio in this composite is about 45/55 corresponding to a capacity ratio, in terms of mAh g⁻¹, of about 450/50.

Because of its unique properties we have selected this Sn−C composite as the preferred anode material for this work. The anode is basically similar to that previously reported,(12) however, considerably upgraded in terms of surface morphology and rate capability. In particular, the issue of large irreversible capacity that affected our original material has here been successfully addressed by a suitable surface treatment;(13) see also Experimental Section.

Figure 1, which compares the voltage profiles of the first charge−discharge cycle of a lithium cell using a pristine Sn−C electrode (A) and a treated Sn−C electrode (B), demonstrates the beneficial effect of the treatment. A considerably large irreversible capacity (due to the occurrence of side reactions involving electrolyte oxidation with formation of a solid electrolyte interface, SEI) amounting to 63% is shown in the first cycle, after which the electrode assumes the expected steady-state behavior, namely a stable reversible capacity of the order of 500 mAh g⁻¹ associated with the electrochemical process involving the alloying of lithium in tin:

Following the surface treatment, the electrode maintains its high capacity response, however, with a remarkable reduction of the irreversible capacity, that passed from ca. 680 mAh g⁻¹ (63% of irreversibility) to ca. 65 mAh g⁻¹(14% of irreversibility); see Figure 1B.

Figure 1. Voltage profiles of a pristine (A) and of a surface-treated (B) Sn−C electrode in a lithium cell. The inset shows the differencesin cycle behavior of the two electrodes. Cycling current 80 mA g⁻¹,voltage limits 0.01−2 V. For the treatment procedure, see Experimental Section.

The Sn−C electrode has been also upgraded in terms of rate capability. Improvement in the morphology, i.e., assuring a uniform distribution of the nanometric tin particles in the amorphous carbon matrix and avoiding any aggregation, allowed the electrode to operate under high current rates. The high resolution transmission electron microscopy (HRTEM) image of the upgraded Sn−C electrode in Figure 2A clearly shows that the tin nanoparticles are uniformly dispersed in the carbon matrix. Figure 2B shows the typical response of the Sn−C electrode in a lithium cell, demonstrating that this electrode can indeed operate under a high rate and still keep a high percentage of the maximum capacity, i.e. 300 mAh g⁻¹, at a current of 600 mA g⁻¹ (1.2C rate).

Figure 2. High resolution transmission electron microscopy (HRTEM) images

(A) and galvanostatic cycling test in a lithium cell at various currents 

(B) of the optimized Sn−C nanocomposite anode.

3.2 The Cathode

For the progress of lithium ion battery technology, it is highly desirable to replace the common, high cost lithium cobalt oxide with cathodes based on more abundant and more environmentally friendly materials. Good choices are lithium manganese spinels. However, the response of these alternative cathodes in lithium cells is strongly influenced by the particle size and by the presence of doping metals.(14, 15) Reduction in the particle size, on one hand, significantly improves the kinetics of the electrochemical lithium insertion/extraction reactions, but on the other hand, increases reactivity for the electrolyte decomposition.(14)

In this work we have addressed this contradictory issue by doping LiMn₂O₄ spinel with Ni and Co and, at the same time, by preparing the resulting Li[Ni_0.45Co_0.1Mn_1.45]O₄ cathode with particles at micrometric size (in order to avoid electrolyte decomposition) and using a metal ratio that is expected to provide high working voltage and high rate capability. Figure 3 shows the XRD patterns of our Li[Ni_0.45Co_0.1Mn_1.45]O₄ material. The pattern confirms the well-crystallized cubic spinel structure with Fd3m space group (JCPDS: #802162).

Figure 3. X-ray diffraction patterns of the Li[Ni_0.45Co_0.1Mn_1.45]O₄ spinel electrode.

Figure 4 shows he field emission scanning electron microscopy (FESEM) images of the optimized Li[Ni_0.45Co_0.1Mn_1.45]O₄ material. Image A clearly reveals that the particle size is effectively of the micrometer order, and image B that these particles are organized in aggregates with a highly uniform distribution and shape. This particular morphology is assumed to be the most favorable in terms of electrode performance because it assures that a high tap density is maintained at a macroscopic level, and a low particle size at a microscopic level, as indeed demonstrated by the electrochemical tests, see below.

Figure 4. Field emission scanning electron microscopy (FESEM) images of the Li[Ni_0.45Co_0.1Mn_1.45]O₄ spinel electrode.

The electrochemical response of the Li[Ni_0.45Co_0.1Mn_1.45]O₄ electrode in a lithium cell was first investigated by potentiodinamic cycling with galvanostatic acceleration (PCGA), i.e., a very powerful quasiequilibrium technique that provides useful, quasithermodynamic information on the characteristics of the electrochemical process.(16, 18)



Figure 5 shows the PCGA response of the Li[Ni_0.45Co_0.1Mn_1.45]O₄ electrode in terms of potential versus step capacity δQ. The voltage profile of the electrochemical process of our substituted manganese cathode:namely the lithium removal (charge)−lithium uptake (discharge) into the spinel structure involves typically one lithium equivalent per mole with an associated theoretical capacity of 132 mAh g⁻¹. Figure 5 shows that the charge process evolves with two high voltage oxidation peaks, at 4.66 V and 4.74 V vs Li, respectively. The process is reversed in discharge in two reduction peaks at a potential value of 4.69 V and 4.61 V vs Li, respectively. These peaks are most likely associated with the Ni⁴⁺/Ni³⁺ and Ni³⁺/Ni²⁺ redox couples.(14) In addition, Figure 5 reveals differences in profile and in the involved capacity when passing from the first cycle to the following one where the response overlaps with excellent reversibility. It is assumed that this initial irreversibility is due to a series of side reactions involving electrolyte oxidation with solid electrolyte interface (SEI) formation, as well as initial reorganization of the spinel structure.(14, 19)

Figure 5. Potentiodynamic cycling with galvanostatic acceleration (PCGA) response of the Li[Ni_0.45Co_0.1Mn_1.45]O₄ electrode in a lithium cell. Stepwise potential scans 5 mV, minimum current limit of 10 μA, potential limits 3.5−4.9 V vs Li. Electrolyte EC:EMC 3:7, LiPF₆ 1.2 M.

Figure 6A shows the cycling response of the Li[Ni_0.45Co_0.1Mn_1.45]O₄ electrode in a lithium cell, while the inset illustrates the voltage profile of the first charge−discharge cycle. The working voltage, around 4.7−4.8 V, reflects the expected electrochemical process, see eq 2. The specific capacity of the first charge (150 mAhg⁻¹) is higher than the theoretical value (132 mAh g⁻¹) due to the above-discussed initial irreversible process. Under steady-state conditions, the electrode assumes a capacity of 130 mAh g⁻¹ and keeps 95.4% of this value for a life extending to 200 cycles. This excellent cycle life and the high operation voltage confirm the validity of our approach in terms of choice of the appropriate morphology and type and condition of doping metals.

Figure 6B shows the electrode response at various C-rate currents. The figure shows the excellent rate-capability of the electrode that can operate at a rate as high as 10C, still keeping 94% of the initial capacity. Again, this result supports the rationality of our approach.

Figure 6. Cycling performance of the galvanostatic test run at 0.5C-rate (A) and at various C-rates (B) of a Li[Ni_0.45Co_0.1Mn_1.45]O₄ electrode in a lithium cell (1C = 132 mAh g⁻¹). The inset shows the voltage vs specific capacity profiles of the first charge−discharge cycle. Electrolyte EC:EMC 3:7, LiPF₆ 1.2 M.

3.3 Full SnC/Li[Ni_0.45Co_0.1Mn_1.45]O₄ Battery

After activation,(10, 13) the Sn−C anode was combined with the Li[Ni_0.45Co_0.1Mn_1.45]O₄ cathode to form a complete lithium ion battery using an ethylene carbonate:ethyl methyl carbonate, EC:EMC, lithium hexafluorophosphate, LiPF₆, electrolyte. The battery was tested by galvanostatic cycle runs at various current rates.

 
Figure7A shows a typical charge−discharge cycle at 1C rate, reflecting the process:

Figure 7B shows the cycling response of the battery. The tests show that the practical working voltage of the battery ranges between 3.9 V and 4.7 V while the specific capacity, related to the cathode mass, is of the order of 125 mAh g⁻¹. In addition, the battery can cycle at 1C with a very stable capacity delivery. Taking an average voltage of 4.2 V (see Figure 7A), a top specific energy density value of 500 Wh kg⁻¹ is obtained. Assuming a 1/3 reduction factor associated with the weight of the electrolyte, current collector, and aluminum case in a pouch configuration, we obtain a 170 Wh kg⁻¹ value that still exceeds that offered by conventional lithium ion batteries chemistry.

Figure
7. Typical charge−discharge cycle (A) and cycling response of SnC/Li[Ni_0.45Co_0.1Mn_1.45]O₄ battery. Voltage limits: 3.1−4.8 V, 1C = 132 mAh g⁻¹. Electrolyte EC:EMC 3:7, LiPF₆ 1.2 M.

Figure 8 shows the response in terms of charge−discharge rates. The battery has high rate capability and can be cycled at 5C, still delivering 110 mAh g⁻¹ with only 14% of capacity decay.

Figure 8. Cycling response at various rates of the SnC/Li[Ni_0.45Co_0.1Mn_1.45]O₄ battery, 1C = 132 mAh g⁻¹. Electrolyte EC:EMC 3:7, LiPF₆ 1.2 M.

Note that the particular electrode morphology adopted here assures not only a battery with stable cycle life and high rate capacity but, due to the high tap density of both electrodes, also an expected high volumetric energy density.

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