TECHNICAL FIELD

The present disclosure relates to a negative electrode active material, a mixed negative electrode active material, a negative electrode for non-aqueous electrolyte secondary batteries, a lithium ion secondary battery, a production method of negative electrode active material, and a production method of lithium ion secondary battery.

BACKGROUND ART

In recent years, small-sized, electronic devices represented by mobile terminals and the like have been widely spread, and further down-sizing, lighter weight and longer life are strongly demanded. To a market demand like this, secondary batteries capable of obtaining, in particular, a smaller size, a lighter weight and a higher energy density have been forwarded. The secondary batteries have been studied to apply also to, without limiting to small-sized electronic devices, large-sized electronic devices represented by automobiles and power-storage systems represented by houses or the like.

Among these, a lithium ion secondary battery is highly expected because a smaller size and higher capacity are likely to be obtained and the energy density higher than that of a lead battery or a nickel-cadmium battery may be obtained.

The lithium ion secondary battery includes a positive electrode and a negative electrode, and an electrolytic solution together with a separator. The negative electrode includes a negative electrode active material related to a charge/discharge reaction.

As the negative electrode active material, while a carbon material is widely used, a further improvement in a battery capacity is demanded from recent market demand. In order to improve the battery capacity, it has been studied to use silicon as the negative electrode active material. This is because since silicon has a theoretical capacity (4199 mAh/g) of 10 times or more a theoretical capacity of graphite (372 mAh/g), a great improvement in the battery capacity is expected. A development of a silicon material as the negative electrode active material includes studies on not only a silicon simple substance but also compounds represented by alloys, oxides or the like. Furthermore, shapes of the active material have been studied, regarding the carbon material, from a standard coating type to an integrated type directly deposited on a current collector.

However, when the silicon is used as a main raw material as the negative electrode active material, since the negative electrode active material expands and contracts during charge/discharge, mainly the neighborhood of a superficial layer of the negative electrode active material tends to be broken. Furthermore, an ionic substance is generated inside the active material, and the negative electrode active material tends to be broken. When a superficial layer of the negative electrode active material is broken, a new surface is generated thereby, and a reaction area of the active material increases. At this time, since a decomposition reaction of an electrolytic solution occurs on the new surface and a film that is a decomposition product, of the electrolytic solution is formed, on the new surface, therefore the electrolytic solution is consumed. Therefore, the cycle characteristics tend to be degraded.

Until now, in order to improve an initial efficiency and the cycle characteristics of a battery, negative electrode materials for lithium ion secondary batteries having the silicon material as a main material and electrode configurations have been variously studied.

Specifically, in order to obtain excellent, cycle characteristics and high safety, silicon and amorphous silicon dioxide are simultaneously deposited by using a gas phase method (see, for example, Patent Literature 1 below). Furthermore, in order to obtain high battery capacity and safety, a carbon material (an electron conducting material) is provided, on a superficial layer of particles of silicon oxide (see, for example, Patent Literature 2 below). Furthermore, in order to improve the cycle characteristics and to obtain high input/output characteristics, an active material containing silicon and oxygen is prepared and an active material layer having a high oxygen ratio in the neighborhood of a current collector is formed (see, for example, Patent Literature 3 below). Still further more, in order to improve the cycle characteristics, oxygen is contained in a silicon active material such that an average oxygen content is 40 at. % or less, and an oxygen content is high in a place close to a current collector (see, for example, Patent Literature 4 below).

Furthermore, in order to improve a first time charge/discharge efficiency, a nano composite containing a Si phase, SiO2and a MyO metal oxide is used (see, for example, Patent Literature 5, below). Still furthermore, in order to improve the cycle characteristics, SiOx(0.8≤x≤1.5, a particle size range=1 μm to 50 μm) and a carbon material are mixed and sintered at a high temperature (see, for example, Patent Literature 6, below). Furthermore, in order to improve the cycle characteristics, an active material is controlled such that a mole ratio of oxygen to silicon in a negative electrode active material is set to from 0.1 to 1.2, and, in the neighborhood of an interface of the active material and a current collector, a difference of a maximum value and a minimum value of the molar ratios is in the range of 0.4 or less (see, for example, Patent Literature 7, below). Still furthermore, in order to improve battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Literature 8, below). Furthermore, in order to improve the cycle characteristics, a hydrophobic layer such as a silane compound is formed on a superficial layer of a silicon material (see, for example, Patent Literature 9, below). Still furthermore, in order to improve the cycle characteristics, a silicon oxide is used, and a graphite film is formed on a superficial layer thereof to impart electric conductivity (see, for example, Patent Literature 10, below). In the Patent Literature 10, regarding a shift value obtained from a Raman spectrum of the graphite film, broad peaks appear at 1330 cm−1and 1580 cm−1, and an intensity ratio thereof I1330/I1580is 1.5<I1330/I1580<3. Furthermore, in order to improve high battery capacity and cycle characteristics, particles having a silicon crystallite phase dispersed in silicon dioxide are used (see, for example, Patent Literature 11, below). Still furthermore, in order to improve overcharge and overdischarge characteristics, silicon oxide in which an atomic ratio of silicon and oxygen is controlled to 1:y (0<y<2) is used (see, for example, Patent Literature 12, below).

CITATION LIST

Patent Literature

Patent Literature 1: JP 2001-185127 APatent Literature 2: JP 2002-042806 APatent Literature 3: JP 2006-164954 APatent Literature 4: JP 2006-114454 APatent Literature 5: JP 2009-070825 APatent Literature 6: JP 2008-282819 APatent Literature 7: JP 2008-251369 APatent Literature 8: JP 2008-177346 APatent Literature 9: JP 2007-234255 APatent Literature 10: JP 2009-212074 APatent Literature 11: JP 2009-205950 APatent Literature 12: JP 2997741 B1

SUMMARY OF INVENTION

Technical Problem

As was described above, in recent years, higher performance and multi-functionalization of small-sized mobile devices typical in the electronic devices have been forwarded, and a lithium ion secondary battery that is a main power source thereof is required to have an increased battery capacity. As one measure to solve the problem, it is desired to develop a lithium ion secondary battery formed of a negative electrode that uses a silicon-based active material as a main material. Furthermore, the lithium ion secondary battery that uses the silicon-based active material is desired to have the first time efficiency and cycle characteristics close to equal with those of a lithium ion secondary battery that uses a carbon-based active material. However, a negative electrode active material that shows the same first time efficiency and cycle stability as those in the lithium ion secondary battery that uses a carbon-based active material has not been proposed.

The present invention was performed in view of the above problems, and it is intended to provide a negative electrode active material, a mixed negative electrode active material containing the negative electrode active material, a negative electrode for nonaqueous electrolyte secondary batteries containing the mixed negative electrode active material, and a lithium ion secondary battery, which are capable of increasing a battery capacity and improving the cycle characteristics and the initial charge/discharge characteristics when used as the negative electrode active material of a lithium ion secondary battery.

Furthermore, the present disclosure intends to provide a production method of a negative electrode active material having excellent battery characteristics such as described above and a production method of lithium ion secondary batteries.

Solution to Problem

In order to achieve the above object, the present-disclosure is a negative electrode active material comprising particles of negative electrode active material, the particles of negative electrode active material contain particles of silicon compound containing a silicon compound (SiOx:0.5≤x≤6), the particles of silicon compound have, as chemical shift values obtained from a29Si-MAS-NMR spectrum, an intensity A of a peak derived from amorphous silicon (also described as a-Si) obtained in −40 to −60 ppm, an intensity B of a peak derived from silicon dioxide obtained in the vicinity of −110 ppm, and an intensity C of a peak derived from Si obtained in the vicinity of −83 ppm, which satisfy the following formula 1 and formula 2.
B≤1.5×A(1)
B<C(2)

Thus, when satisfying the formula (1), that is, when, a silicon dioxide component is sufficiently small relative to an amorphous silicon component, during charge of a battery, a negative electrode active material having a high battery initial efficiency is formed because an irreversible component generated due to a reaction of a part of silicon dioxide with lithium may be sufficiently-reduced. In addition to this, when satisfying the formula 2, that, is, when the silicon dioxide component is sufficiently small relative to a silicon component, in the same manner as the above, a negative electrode active material having higher battery initial efficiency is formed because the irreversible component generated during charge of the battery may be more reduced. Therefore, the negative electrode active material of the present disclosure has a large battery capacity and may improve the initial efficiency and the cycle characteristics.

At this time, the particles of negative electrode active material preferably contain at least one kind or more of Li2SiO3and Li4SiO4.

Since the negative electrode active material like this is obtained by modifying, in advance, a SiO2component part that becomes unstable during insertion/release of lithium during charge/discharge of the battery in the silicon compound to a relatively stable lithium silicate, the irreversible capacitance generated during charge may be reduced.

Furthermore, the chemical shift value obtained from the29Si-MAS-NMR spectrum preferably has a peak in the vicinity of −130 ppm.

The peak of which chemical shift value appears in the vicinity of −130 ppm is also considered derived from amorphous silicon. Therefore, since a ratio of the amorphous silicon component is larger, the initial efficiency and cycle characteristics may be more improved.

Furthermore, the particles of silicon compound preferably have a half value width (2θ) of a diffraction peak derived from a Si (111) crystal plane obtained by X-ray diffractometry of 1.2° or more and a crystallite size corresponding to the crystal plane of 7.5 nm or less.

When the negative electrode active material in which the particles of silicon compound have the silicon crystallinity is used as the negative electrode active material of a lithium ion secondary battery, more excellent cycle characteristics and initial charge/discharge characteristics may be obtained.

Furthermore, when a test cell formed of a negative electrode containing a mixture of the negative electrode active material and a carbon-based active material and a counter electrode lithium is prepared, in the test cell, charge/discharge formed of charge during which a current is flowed so as to insert lithium into the negative electrode active material and discharge during which the current is flowed so as to detach lithium from the negative electrode active material is carried out 30 times, and when a graph showing a relationship between a differential value dQ/dV obtained by differentiating a discharge capacity Q by a potential V of the negative electrode based on the counter electrode lithium and the potential V is depicted in each charge/discharge, the potential V of the negative electrode during discharges on or after X times (1≤X≤30) preferably has a peak in the range of from 0.40 V to 0.55 V.

Since the above peak in the V-dQ/dV curve is similar to the peak of the silicon material and has a sharp rise of a discharge curve on a higher potential side, when a battery is designed, a capacity is easily developed. Furthermore, when the peak is developed during the charge/discharge within 30 times, the negative electrode active material in which a stable bulk is formed is obtained.

Furthermore, a median diameter of the particles of negative electrode active material is preferably 1.0 μm or more and 15 μm or less.

When the median diameter is 1.0 μm or more, an increase of the battery irreversible capacity may be suppressed from increasing due to an increase in a surface area a mass. On the other hand, by setting the median diameter to 15 μm or less, the particles become difficult to be cracked and a new surface is difficult to be developed.

Furthermore, the particles of negative electrode active material preferably contain a carbon material on a superficial part.

Like this, when the particles of negative electrode active material contain the carbon material on the superficial part thereof, the electric conductivity may be improved.

Furthermore, an average thickness of the carbon material is preferably 10 nm or more and 5000 nm or less.

When the average thickness of the carbon material is 10 nm or more and 5000 nm or less, the electric conductivity may be improved. Furthermore, when an average thickness of the carbon material to be coated is 5000 nm or less, by using the negative electrode active material containing the particles of negative electrode active material like this in a lithium ion secondary battery, the battery capacitance may be suppressed from degrading because the particles of silicon compound may be sufficiently secured.

A mixed negative electrode active material characterized by containing the negative electrode active material and a carbon-based active material is provided.

Thus, when the carbon-based active material is contained together with the negative electrode active material (silicon-based negative electrode active material) of the present disclosure as a material forming a negative electrode active material layer, the electric conductivity of the negative electrode active material layer may be improved and an expansion stress accompanying the charge may be alleviated. Furthermore, by mixing the silicon-based negative electrode active material with the carbon-based active material, the battery capacity may be improved.

Furthermore, in order to achieve the object, the present disclosure provides a negative electrode for nonaqueous electrolyte secondary batteries, characterized by containing the mixed negative electrode active material and by having a ratio of a mass of the negative electrode active material relative to a total mass of the negative electrode active material and the carbon-based active material of 6% by mass or more.

When the ratio of mass of the negative electrode active material (silicon-based negative electrode active material) relative to a total amount of masses of the negative electrode active material (silicon-based negative electrode active material) and the carbon-based active material is 6% by mass or more, the battery capacity may be further improved.

Furthermore, in order to achieve the object, the present disclosure provides a negative electrode for nonaqueous electrolyte secondary batteries, characterized by having a negative electrode active material layer formed with the mixed negative electrode active material and a negative electrode current collector, having the negative electrode active material layer formed on the negative electrode current collector, containing carbon and sulfur in the negative electrode current collector, and having a content, of each thereof of 70 mass ppm or less.

Thus, when the negative electrode current collector constituting the negative electrode contains the carbon and sulfur in the amount described above, the negative electrode may be suppressed from being deformed during the charge.

Furthermore, in order to achieve the object, the present disclosure provides a lithium ion secondary battery characterized by using negative electrode containing the negative electrode active material.

When the lithium ion secondary battery uses the negative electrode containing the negative electrode active material like this, high, capacity, excellent cycle characteristics and initial charge/discharge characteristics are obtained.

Furthermore, in order to achieve the object, the present disclosure is a method of producing a negative electrode active material containing particles of negative electrode active material containing particles of silicon compound, the method including preparing particles of negative electrode active material containing particles of silicon compound containing a silicon compound (SiOx:0.5≤x≤1.6), and selecting from the particles of negative electrode active material, particles of negative electrode active material in which, as chemical shift values obtained from a29Si-MAS-NMR spectrum, an intensity A of a peak derived from amorphous silicon obtained in −4.0 to −60 ppm, an intensity B of a peak derived from a silicon dioxide region obtained in the vicinity of −110 ppm, and an intensity C of a peak derived from Si obtained in the vicinity of −83 ppm satisfy the following formula 1 and formula 2, wherein producing the negative electrode active material with the selected particles of negative electrode active material.
B≤1.5×A(1)
B<C(2)

When the negative electrode active material is produced by selecting thus the particles of silicon-based active material, the negative electrode active material having high capacity, excellent cycle characteristics and initial charge/discharge characteristics when used as the negative electrode active material of the lithium ion secondary battery may be produced.

Furthermore, in order to achieve the object, the present disclosure provides a production method of a lithium ion secondary battery, characterized by preparing a negative electrode using the negative electrode active material produced according to the production method of the negative electrode active material, and by producing a lithium ion secondary battery with the prepared negative electrode.

By using the negative electrode active material produced as described above, a lithium ion secondary-battery having high capacity, excellent cycle characteristics and initial charge/discharge characteristics may be produced.

Advantageous Effects of Invention

The negative electrode active material of the present disclosure may obtain high capacity and excellent cycle characteristics and initial charge/discharge characteristics when used as the negative electrode active material of secondary batteries. Furthermore, similar effect may be obtained also in the mixed negative electrode active material, the negative electrode, and the lithium ion secondary battery, which contain the negative electrode active material. Furthermore, according to the production method of negative electrode active material of the present disclosure, when used as the negative electrode active material of the secondary battery, a negative electrode active material having excellent cycle characteristics and initial charge/discharge characteristics may be produced.

